diff --git a/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/migration.sql b/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/migration.sql
new file mode 100644
index 000000000..151ff73c0
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/migration.sql
@@ -0,0 +1,21 @@
+-- CreateEnum
+CREATE TYPE "StripeWebhookEventStatus" AS ENUM ('PENDING', 'PROCESSED', 'FAILED');
+
+-- CreateTable
+CREATE TABLE "StripeWebhookEvent" (
+    "id" UUID NOT NULL,
+    "stripeEventId" TEXT NOT NULL,
+    "eventType" TEXT NOT NULL,
+    "stripeAccountId" TEXT,
+    "payload" JSONB NOT NULL,
+    "status" "StripeWebhookEventStatus" NOT NULL DEFAULT 'PENDING',
+    "lastError" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+    "processedAt" TIMESTAMP(3),
+
+    CONSTRAINT "StripeWebhookEvent_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "StripeWebhookEvent_stripeEventId_key" ON "StripeWebhookEvent"("stripeEventId");
diff --git a/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/tests/unique-stripe-event-id.ts b/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/tests/unique-stripe-event-id.ts
new file mode 100644
index 000000000..8bca6123e
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260622000000_add_stripe_webhook_event/tests/unique-stripe-event-id.ts
@@ -0,0 +1,60 @@
+import { randomUUID } from "crypto";
+import type { Sql } from "postgres";
+import { expect } from "vitest";
+
+export const preMigration = async (sql: Sql) => {
+  // Table does not exist before the migration, so nothing to seed.
+  return {};
+};
+
+export const postMigration = async (sql: Sql) => {
+  const tables = await sql<{ table_name: string }[]>`
+    SELECT table_name
+    FROM information_schema.tables
+    WHERE table_schema = 'public'
+      AND table_name = 'StripeWebhookEvent'
+  `;
+  expect(Array.from(tables)).toMatchInlineSnapshot(`
+    [
+      {
+        "table_name": "StripeWebhookEvent",
+      },
+    ]
+  `);
+
+  const eventId = `evt_${randomUUID()}`;
+
+  await sql`
+    INSERT INTO "StripeWebhookEvent" ("id", "stripeEventId", "eventType", "payload", "updatedAt")
+    VALUES (${randomUUID()}::uuid, ${eventId}, 'invoice.payment_succeeded', '{"id":"evt"}'::jsonb, NOW())
+  `;
+
+  // Status defaults to PENDING.
+  const inserted = await sql`
+    SELECT "status" FROM "StripeWebhookEvent" WHERE "stripeEventId" = ${eventId}
+  `;
+  expect(Array.from(inserted)).toMatchInlineSnapshot(`
+    [
+      {
+        "status": "PENDING",
+      },
+    ]
+  `);
+
+  // The same Stripe event id cannot be inserted twice (idempotency guarantee).
+  await expect(sql`
+    INSERT INTO "StripeWebhookEvent" ("id", "stripeEventId", "eventType", "payload", "updatedAt")
+    VALUES (${randomUUID()}::uuid, ${eventId}, 'invoice.payment_succeeded', '{"id":"evt2"}'::jsonb, NOW())
+  `).rejects.toThrow(/StripeWebhookEvent_stripeEventId_key/);
+
+  // A different event id is fine, and the status enum rejects invalid values.
+  await sql`
+    INSERT INTO "StripeWebhookEvent" ("id", "stripeEventId", "eventType", "payload", "status", "updatedAt")
+    VALUES (${randomUUID()}::uuid, ${`evt_${randomUUID()}`}, 'invoice.paid', '{}'::jsonb, 'PROCESSED', NOW())
+  `;
+
+  await expect(sql`
+    INSERT INTO "StripeWebhookEvent" ("id", "stripeEventId", "eventType", "payload", "status", "updatedAt")
+    VALUES (${randomUUID()}::uuid, ${`evt_${randomUUID()}`}, 'invoice.paid', '{}'::jsonb, 'NOT_A_STATUS', NOW())
+  `).rejects.toThrow();
+};
diff --git a/apps/backend/prisma/schema.prisma b/apps/backend/prisma/schema.prisma
index 1b4d8544f..d152621c6 100644
--- a/apps/backend/prisma/schema.prisma
+++ b/apps/backend/prisma/schema.prisma
@@ -1424,6 +1424,32 @@ model OutgoingRequest {
   @@index([startedFulfillingAt, deduplicationKey])
 }
 
+enum StripeWebhookEventStatus {
+  PENDING
+  PROCESSED
+  FAILED
+}
+
+// Idempotency + recovery log for incoming Stripe webhook events. Each event is
+// persisted synchronously (keyed on the Stripe `event.id`) before we ack 200 to
+// Stripe, so redeliveries are deduped and the full `payload` of any
+// PENDING/FAILED row can be replayed manually. Not tenancy-scoped: the Stripe
+// account -> tenancy resolution happens during processing, not here.
+model StripeWebhookEvent {
+  id String @id @default(uuid()) @db.Uuid
+
+  stripeEventId   String                   @unique
+  eventType       String
+  stripeAccountId String?
+  payload         Json
+  status          StripeWebhookEventStatus @default(PENDING)
+  lastError       String?
+
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+  processedAt DateTime?
+}
+
 // BulldozerStorageEngine is managed externally (see prisma.config.ts
 // `tables.external`). It's created by migrations and interacted with
 // via raw SQL — not through the Prisma client. Keeping it out of the
diff --git a/apps/backend/src/app/api/latest/integrations/stripe/webhooks/route.tsx b/apps/backend/src/app/api/latest/integrations/stripe/webhooks/route.tsx
index 0388a0e80..7656826eb 100644
--- a/apps/backend/src/app/api/latest/integrations/stripe/webhooks/route.tsx
+++ b/apps/backend/src/app/api/latest/integrations/stripe/webhooks/route.tsx
@@ -1,5 +1,7 @@
 import { sendEmailToMany, type EmailOutboxRecipient } from "@/lib/emails";
 import { bulldozerWriteOneTimePurchase } from "@/lib/payments/bulldozer-dual-write";
+import { claimStripeEvent, markStripeEventFailed, markStripeEventProcessed } from "@/lib/stripe-webhook-events";
+import { runAsynchronouslyAndWaitUntil } from "@/utils/background-tasks";
 import { listPermissions } from "@/lib/permissions";
 import { getHexclaveStripe, getStripeForAccount, resolveProductFromStripeMetadata, syncStripeSubscriptions, upsertStripeInvoice } from "@/lib/stripe";
 import type { StripeOverridesMap } from "@/lib/stripe-proxy";
@@ -466,13 +468,29 @@ export const POST = createSmartRouteHandler({
       throw new StatusError(400, "Invalid stripe-signature header");
     }
 
-    try {
-      await processStripeWebhookEvent(event);
-    } catch (error) {
-      captureError("stripe-webhook-receiver", error);
-      throw error;
+    // Persist the event for idempotency + recovery BEFORE acking. Stripe
+    // delivers at-least-once
+    const { shouldProcess } = await claimStripeEvent(event);
+    if (!shouldProcess) {
+      return {
+        statusCode: 200,
+        bodyType: "json",
+        body: { received: true, deduplicated: true },
+      };
     }
 
+    // Ack Stripe immediately and process in the background.
+    // Stripe recommends ACKing ASAP to avoid timeouts and redeliveries
+    runAsynchronouslyAndWaitUntil(async () => {
+      try {
+        await processStripeWebhookEvent(event);
+        await markStripeEventProcessed(event.id);
+      } catch (error) {
+        captureError("stripe-webhook-receiver", error);
+        await markStripeEventFailed(event.id, error);
+      }
+    });
+
     return {
       statusCode: 200,
       bodyType: "json",
diff --git a/apps/backend/src/lib/stripe-webhook-events.test.ts b/apps/backend/src/lib/stripe-webhook-events.test.ts
new file mode 100644
index 000000000..65aae6ce3
--- /dev/null
+++ b/apps/backend/src/lib/stripe-webhook-events.test.ts
@@ -0,0 +1,87 @@
+import { randomUUID } from "node:crypto";
+import type Stripe from "stripe";
+import { describe, expect, it } from "vitest";
+import { StripeWebhookEventStatus } from "@/generated/prisma/client";
+import { globalPrismaClient } from "@/prisma-client";
+import { claimStripeEvent, markStripeEventFailed, markStripeEventProcessed } from "./stripe-webhook-events";
+
+// Test fixtures only need the fields the helper reads (id/type/account) plus a
+// JSON-serializable body. Building a full Stripe.Event is impractical, so we
+// cast a minimal object — any drift in the fields we actually use is still
+// caught because the helper reads them directly.
+function makeEvent(): Stripe.Event {
+  return {
+    id: `evt_${randomUUID()}`,
+    type: "invoice.payment_succeeded",
+    account: "acct_test_123",
+    data: { object: { id: "in_test", note: "fixture" } },
+  } as unknown as Stripe.Event;
+}
+
+describe("stripe webhook event idempotency (real DB)", () => {
+  it("claims a brand new event and persists it as PENDING", async ({ expect }) => {
+    const event = makeEvent();
+
+    const { shouldProcess } = await claimStripeEvent(event);
+    expect(shouldProcess).toBe(true);
+
+    const row = await globalPrismaClient.stripeWebhookEvent.findUnique({
+      where: { stripeEventId: event.id },
+    });
+    expect(row).not.toBeNull();
+    expect(row?.status).toBe(StripeWebhookEventStatus.PENDING);
+    expect(row?.eventType).toBe(event.type);
+    expect(row?.stripeAccountId).toBe(event.account);
+    expect(row?.processedAt).toBeNull();
+    expect(row?.lastError).toBeNull();
+    // The full event payload is stored so dropped/failed events can be replayed.
+    expect(row?.payload).toMatchObject({ id: event.id, type: event.type });
+  });
+
+  it("allows reprocessing while the prior delivery is still PENDING", async ({ expect }) => {
+    const event = makeEvent();
+
+    const first = await claimStripeEvent(event);
+    expect(first.shouldProcess).toBe(true);
+
+    // Redelivery before the background work finished: we must NOT skip, otherwise
+    // a crash between claim and processing would silently drop the event forever.
+    const second = await claimStripeEvent(event);
+    expect(second.shouldProcess).toBe(true);
+  });
+
+  it("deduplicates once the event has been fully PROCESSED", async ({ expect }) => {
+    const event = makeEvent();
+
+    await claimStripeEvent(event);
+    await markStripeEventProcessed(event.id);
+
+    const processedRow = await globalPrismaClient.stripeWebhookEvent.findUnique({
+      where: { stripeEventId: event.id },
+    });
+    expect(processedRow?.status).toBe(StripeWebhookEventStatus.PROCESSED);
+    expect(processedRow?.processedAt).not.toBeNull();
+    expect(processedRow?.lastError).toBeNull();
+
+    // A Stripe redelivery of an already-processed event must be a no-op.
+    const redelivery = await claimStripeEvent(event);
+    expect(redelivery.shouldProcess).toBe(false);
+  });
+
+  it("records the error on failure and allows recovery via redelivery", async ({ expect }) => {
+    const event = makeEvent();
+
+    await claimStripeEvent(event);
+    await markStripeEventFailed(event.id, new Error("boom while processing"));
+
+    const failedRow = await globalPrismaClient.stripeWebhookEvent.findUnique({
+      where: { stripeEventId: event.id },
+    });
+    expect(failedRow?.status).toBe(StripeWebhookEventStatus.FAILED);
+    expect(failedRow?.lastError).toContain("boom while processing");
+
+    // FAILED rows must reprocess so a manual Stripe "Resend" can recover them.
+    const recovery = await claimStripeEvent(event);
+    expect(recovery.shouldProcess).toBe(true);
+  });
+});
diff --git a/apps/backend/src/lib/stripe-webhook-events.ts b/apps/backend/src/lib/stripe-webhook-events.ts
new file mode 100644
index 000000000..0d1e0124d
--- /dev/null
+++ b/apps/backend/src/lib/stripe-webhook-events.ts
@@ -0,0 +1,69 @@
+import { Prisma, StripeWebhookEventStatus } from "@/generated/prisma/client";
+import { globalPrismaClient } from "@/prisma-client";
+import { errorToNiceString, HexclaveAssertionError } from "@hexclave/shared/dist/utils/errors";
+import type Stripe from "stripe";
+
+/**
+ * Idempotency + recovery layer for incoming Stripe webhook events.
+ *
+ * Each event is persisted (keyed on the Stripe `event.id`) synchronously before
+ * we ack 200 to Stripe. Processing then runs in the background. Because Stripe
+ * delivers at-least-once, this is what guarantees the receipt fan-out happens at
+ * most once per event. The full `payload` is stored so PENDING/FAILED rows can
+ * be replayed manually if the background work is dropped (e.g. instance recycle).
+ */
+
+/**
+ * Records the event (or detects a prior one) and decides whether processing
+ * should run. Returns `shouldProcess: false` only when the event has already
+ * been fully PROCESSED — PENDING/FAILED rows are allowed to reprocess so that a
+ * manual Stripe "Resend" can recover a dropped/failed event.
+ */
+export async function claimStripeEvent(event: Stripe.Event): Promise<{ shouldProcess: boolean }> {
+  try {
+    await globalPrismaClient.stripeWebhookEvent.create({
+      data: {
+        stripeEventId: event.id,
+        eventType: event.type,
+        stripeAccountId: event.account ?? null,
+        payload: JSON.parse(JSON.stringify(event)) as Prisma.InputJsonValue,
+        status: StripeWebhookEventStatus.PENDING,
+      },
+    });
+    return { shouldProcess: true };
+  } catch (error) {
+    // Unique violation on stripeEventId => we've seen this event before.
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === "P2002") {
+      const existing = await globalPrismaClient.stripeWebhookEvent.findUnique({
+        where: { stripeEventId: event.id },
+      });
+      return { shouldProcess: existing?.status !== StripeWebhookEventStatus.PROCESSED };
+    }
+
+    throw new HexclaveAssertionError(
+      `Failed to claim Stripe webhook event for idempotency: ${event.id}`,
+      { cause: error, stripeEventId: event.id, eventType: event.type },
+    );
+  }
+}
+
+export async function markStripeEventProcessed(stripeEventId: string): Promise<void> {
+  await globalPrismaClient.stripeWebhookEvent.update({
+    where: { stripeEventId },
+    data: {
+      status: StripeWebhookEventStatus.PROCESSED,
+      processedAt: new Date(),
+      lastError: null,
+    },
+  });
+}
+
+export async function markStripeEventFailed(stripeEventId: string, error: unknown): Promise<void> {
+  await globalPrismaClient.stripeWebhookEvent.update({
+    where: { stripeEventId },
+    data: {
+      status: StripeWebhookEventStatus.FAILED,
+      lastError: errorToNiceString(error),
+    },
+  });
+}
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/stripe-webhooks.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/stripe-webhooks.test.ts
index 82de0dbdd..aa29a8f19 100644
--- a/apps/e2e/tests/backend/endpoints/api/v1/stripe-webhooks.test.ts
+++ b/apps/e2e/tests/backend/endpoints/api/v1/stripe-webhooks.test.ts
@@ -1,9 +1,42 @@
+import { randomUUID } from "node:crypto";
 import { throwErr } from "@hexclave/shared/dist/utils/errors";
 import { wait } from "@hexclave/shared/dist/utils/promises";
 import { it } from "../../../../helpers";
 import { Auth, bumpEmailAddress, niceBackendFetch, Payments, Project, Team } from "../../../backend-helpers";
 import { getOutboxEmails } from "./emails/email-helpers";
 
+// Stripe webhook events are now deduplicated globally by their `event.id` (see
+// the StripeWebhookEvent table). The dev DB is NOT reset between test runs, so
+// every claimed event needs a per-run unique id, otherwise a second run would
+// hit the dedupe path and skip processing.
+function uniqueEventId(prefix: string) {
+  return `evt_${prefix}_${randomUUID()}`;
+}
+
+// Webhook processing now happens in the background after a fast 200 ack, so DB
+// state is eventually-consistent from the test's perspective. Poll instead of
+// reading immediately after the webhook returns.
+async function waitForItemQuantity(
+  args: { customerType: "user" | "team", customerId: string, itemId: string, expected: number },
+) {
+  let last: number | undefined;
+  for (let i = 0; i < 30; i++) {
+    const res = await niceBackendFetch(
+      `/api/latest/payments/items/${args.customerType}/${args.customerId}/${args.itemId}`,
+      { accessType: "client" },
+    );
+    if (res.status !== 200) {
+      throw new Error(`Unexpected ${res.status} reading item ${args.itemId}`);
+    }
+    last = res.body.quantity;
+    if (last === args.expected) {
+      return;
+    }
+    await wait(500);
+  }
+  throw new Error(`Item ${args.itemId} quantity never reached ${args.expected} (last seen: ${last})`);
+}
+
 async function waitForOutboxEmail(subject: string) {
   for (let i = 0; i < 30; i++) {
     const emails = await getOutboxEmails({ subject });
@@ -26,14 +59,20 @@ async function waitForNoOutboxEmail(subject: string) {
 }
 
 
-it("rejects signed mock_event.succeeded webhook", async ({ expect }) => {
+it("acks unknown signed webhook types (errors handled in background)", async ({ expect }) => {
+  // We now persist + ack the event synchronously and process it in the
+  // background, so an unknown type no longer surfaces a 500 to Stripe. The
+  // "Unknown stripe webhook type" error is captured async and the event row is
+  // marked FAILED (covered deterministically in stripe-webhook-events.test.ts).
   const payload = {
-    id: "evt_test_1",
+    id: uniqueEventId("mock_event_succeeded"),
     type: "mock_event.succeeded",
     account: "acct_test123",
     data: { object: { customer: "cus_test123", metadata: {} } },
   };
-  await expect(Payments.sendStripeWebhook(payload)).rejects.toThrow(/Unknown stripe webhook type received/);
+  const res = await Payments.sendStripeWebhook(payload);
+  expect(res.status).toBe(200);
+  expect(res.body).toEqual({ received: true });
 });
 
 it("returns 400 on invalid signature", async ({ expect }) => {
@@ -53,15 +92,17 @@ it("returns 400 on invalid signature", async ({ expect }) => {
   `);
 });
 
-it("returns 500 on unknown webhook type", async ({ expect }) => {
+it("acks unknown webhook types with 200 (errors handled in background)", async ({ expect }) => {
   const payload = {
-    id: "evt_test_unknown",
+    id: uniqueEventId("unknown_event"),
     type: "unknown.event",
     account: "acct_test123",
     data: { object: {} },
   };
 
-  await expect(Payments.sendStripeWebhook(payload)).rejects.toThrow(/Unknown stripe webhook type received/);
+  const res = await Payments.sendStripeWebhook(payload);
+  expect(res.status).toBe(200);
+  expect(res.body).toEqual({ received: true });
 });
 
 it("returns 400 when signature header is missing (schema validation)", async ({ expect }) => {
@@ -86,7 +127,7 @@ it("accepts chargeback webhooks", async ({ expect }) => {
   const accountId: string = accountInfo.body.account_id;
 
   const payload = {
-    id: "evt_chargeback_test",
+    id: uniqueEventId("chargeback"),
     type: "charge.dispute.created",
     account: accountId,
     data: {
@@ -172,7 +213,7 @@ it("deduplicates one-time purchase on payment_intent.succeeded retry", async ({
   const fullCode = purchaseUrl.split("/purchase/")[1];
   const stackTestTenancyId = fullCode.split("_")[0];
   const payloadObj = {
-    id: "evt_retry_test",
+    id: uniqueEventId("retry"),
     type: "payment_intent.succeeded",
     account: accountId,
     data: {
@@ -198,18 +239,18 @@ it("deduplicates one-time purchase on payment_intent.succeeded retry", async ({
   };
   const res = await Payments.sendStripeWebhook(payloadObj);
   expect(res.status).toBe(200);
-  expect(res.body).toEqual({ received: true });
+  expect(res.body.received).toBe(true);
+
+  // First grant must land before we redeliver, so the duplicate deterministically
+  // hits the event-dedupe path (PROCESSED) rather than racing in-flight work.
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 1 });
 
   const res2 = await Payments.sendStripeWebhook(payloadObj);
   expect(res2.status).toBe(200);
-  expect(res2.body).toEqual({ received: true });
+  expect(res2.body).toEqual({ received: true, deduplicated: true });
 
-  // After duplicate deliveries, quantity should reflect a single OneTimePurchase grant
-  const getAfter = await niceBackendFetch(`/api/latest/payments/items/user/${userId}/${itemId}`, {
-    accessType: "client",
-  });
-  expect(getAfter.status).toBe(200);
-  expect(getAfter.body.quantity).toBe(1);
+  // After the deduplicated redelivery, quantity stays at a single grant.
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 1 });
 });
 
 it("sends a payment receipt email for one-time purchases", async ({ expect }) => {
@@ -268,7 +309,7 @@ it("sends a payment receipt email for one-time purchases", async ({ expect }) =>
   const receiptLink = "https://example.com/receipt/pi_test_receipt_1";
   const paymentIntentId = "pi_test_receipt_1";
   const payloadObj = {
-    id: "evt_receipt_test_1",
+    id: uniqueEventId("receipt"),
     type: "payment_intent.succeeded",
     account: accountId,
     data: {
@@ -313,6 +354,117 @@ it("sends a payment receipt email for one-time purchases", async ({ expect }) =>
   `);
 });
 
+it("sends exactly one receipt when Stripe redelivers the same event", async ({ expect }) => {
+  // Regression test for the duplicate-receipt bug: Stripe delivers at-least-once,
+  // and slow synchronous processing used to time out and trigger redeliveries,
+  // each re-sending the receipt fan-out. The StripeWebhookEvent dedupe must keep
+  // the fan-out to exactly once per event id.
+  const projectDisplayName = `Receipt Idempotency ${randomUUID()}`;
+  await Project.createAndSwitch({ display_name: projectDisplayName });
+  await Payments.setup();
+
+  const itemId = "idem-receipt-credits";
+  const productId = "idem-receipt-ot";
+  const product = {
+    displayName: "Idem Receipt Pack",
+    customerType: "user",
+    serverOnly: false,
+    stackable: true,
+    prices: { one: { USD: "500" } },
+    includedItems: { [itemId]: { quantity: 1 } },
+  };
+
+  await Project.updateConfig({
+    payments: {
+      items: {
+        [itemId]: { displayName: "Credits", customerType: "user" },
+      },
+      products: {
+        [productId]: product,
+      },
+    },
+  });
+
+  const mailbox = await bumpEmailAddress();
+  const { userId } = await Auth.fastSignUp({
+    primary_email: mailbox.emailAddress,
+    primary_email_verified: true,
+  });
+
+  const accountInfo = await niceBackendFetch("/api/latest/internal/payments/stripe/account-info", {
+    accessType: "admin",
+  });
+  expect(accountInfo.status).toBe(200);
+  const accountId: string = accountInfo.body.account_id;
+
+  const createUrlResponse = await niceBackendFetch("/api/latest/payments/purchases/create-purchase-url", {
+    method: "POST",
+    accessType: "client",
+    body: {
+      customer_type: "user",
+      customer_id: userId,
+      product_id: productId,
+    },
+  });
+  expect(createUrlResponse.status).toBe(200);
+  const purchaseUrl = (createUrlResponse.body as { url: string }).url;
+  const fullCode = purchaseUrl.split("/purchase/")[1];
+  const stackTestTenancyId = fullCode.split("_")[0];
+
+  const receiptLink = "https://example.com/receipt/pi_idem_receipt";
+  const eventId = uniqueEventId("idem_receipt");
+  const payloadObj = {
+    id: eventId,
+    type: "payment_intent.succeeded",
+    account: accountId,
+    data: {
+      object: {
+        id: "pi_idem_receipt",
+        customer: userId,
+        amount_received: 500,
+        currency: "usd",
+        charges: { data: [{ receipt_url: receiptLink }] },
+        stack_stripe_mock_data: {
+          "accounts.retrieve": { metadata: { tenancyId: stackTestTenancyId } },
+          "customers.retrieve": { metadata: { customerId: userId, customerType: "USER" } },
+          "subscriptions.list": { data: [] },
+        },
+        metadata: {
+          productId,
+          product: JSON.stringify(product),
+          customerId: userId,
+          customerType: "user",
+          purchaseQuantity: "1",
+          purchaseKind: "ONE_TIME",
+          priceId: "one",
+        },
+      },
+    },
+  };
+
+  const first = await Payments.sendStripeWebhook(payloadObj);
+  expect(first.status).toBe(200);
+  expect(first.body).toEqual({ received: true });
+
+  // Wait for the receipt to land, which proves the first event finished
+  // processing (and is now PROCESSED), so the redeliveries deterministically
+  // take the dedupe path.
+  const subject = `Your receipt from ${projectDisplayName}`;
+  await waitForOutboxEmail(subject);
+
+  for (let i = 0; i < 2; i++) {
+    const redelivery = await Payments.sendStripeWebhook(payloadObj);
+    expect(redelivery.status).toBe(200);
+    expect(redelivery.body).toEqual({ received: true, deduplicated: true });
+  }
+
+  // Give any (incorrectly) re-triggered fan-out a chance to show up, then assert
+  // there is still exactly one receipt email for this project.
+  await wait(1500);
+  const receipts = await getOutboxEmails({ subject });
+  expect(receipts.length).toBe(1);
+});
+
 it("sends a payment failed email for invoice.payment_failed", async ({ expect }) => {
   const projectDisplayName = "Payments Failed Email Test";
   await Project.createAndSwitch({ display_name: projectDisplayName });
@@ -365,7 +517,7 @@ it("sends a payment failed email for invoice.payment_failed", async ({ expect })
   const invoiceId = "in_test_failed_1";
   const invoiceUrl = "https://example.com/billing/update";
   const payloadObj = {
-    id: "evt_invoice_failed_1",
+    id: uniqueEventId("invoice_failed"),
     type: "invoice.payment_failed",
     account: accountId,
     data: {
@@ -458,7 +610,7 @@ it("skips payment failed email when invoice is not uncollectible", async ({ expe
   const invoiceId = "in_test_failed_open_1";
   const invoiceUrl = "https://example.com/billing/open";
   const payloadObj = {
-    id: "evt_invoice_failed_open_1",
+    id: uniqueEventId("invoice_failed_open"),
     type: "invoice.payment_failed",
     account: accountId,
     data: {
@@ -569,7 +721,7 @@ it("syncs subscriptions from webhook and is idempotent", async ({ expect }) => {
   };
 
   const payloadObj = {
-    id: "evt_sub_sync_1",
+    id: uniqueEventId("sub_sync"),
     type: "invoice.paid",
     account: accountId,
     data: {
@@ -586,23 +738,16 @@ it("syncs subscriptions from webhook and is idempotent", async ({ expect }) => {
 
   const res = await Payments.sendStripeWebhook(payloadObj);
   expect(res.status).toBe(200);
-  expect(res.body).toEqual({ received: true });
+  expect(res.body.received).toBe(true);
 
-  const getAfter1 = await niceBackendFetch(`/api/latest/payments/items/user/${userId}/${itemId}`, {
-    accessType: "client",
-  });
-  expect(getAfter1.status).toBe(200);
-  expect(getAfter1.body.quantity).toBe(1);
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 1 });
 
+  // Redelivery of the same event id is deduplicated and leaves state untouched.
   const res2 = await Payments.sendStripeWebhook(payloadObj);
   expect(res2.status).toBe(200);
-  expect(res2.body).toEqual({ received: true });
+  expect(res2.body).toEqual({ received: true, deduplicated: true });
 
-  const getAfter2 = await niceBackendFetch(`/api/latest/payments/items/user/${userId}/${itemId}`, {
-    accessType: "client",
-  });
-  expect(getAfter2.status).toBe(200);
-  expect(getAfter2.body.quantity).toBe(1);
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 1 });
 });
 
 
@@ -682,7 +827,7 @@ it("updates a user's subscriptions via webhook (add then remove)", async ({ expe
   };
 
   const payloadAdd = {
-    id: "evt_sub_add",
+    id: uniqueEventId("sub_add"),
     type: "invoice.paid",
     account: accountId,
     data: {
@@ -701,11 +846,7 @@ it("updates a user's subscriptions via webhook (add then remove)", async ({ expe
   expect(resAdd.status).toBe(200);
   expect(resAdd.body).toEqual({ received: true });
 
-  const afterAdd = await niceBackendFetch(`/api/latest/payments/items/user/${userId}/${itemId}`, {
-    accessType: "client",
-  });
-  expect(afterAdd.status).toBe(200);
-  expect(afterAdd.body.quantity).toBe(1);
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 1 });
 
   const canceledSubscription = {
     ...activeSubscription,
@@ -722,7 +863,7 @@ it("updates a user's subscriptions via webhook (add then remove)", async ({ expe
   };
 
   const payloadRemove = {
-    id: "evt_sub_remove",
+    id: uniqueEventId("sub_remove"),
     type: "customer.subscription.updated",
     account: accountId,
     data: {
@@ -741,11 +882,7 @@ it("updates a user's subscriptions via webhook (add then remove)", async ({ expe
   expect(resRemove.status).toBe(200);
   expect(resRemove.body).toEqual({ received: true });
 
-  const afterRemove = await niceBackendFetch(`/api/latest/payments/items/user/${userId}/${itemId}`, {
-    accessType: "client",
-  });
-  expect(afterRemove.status).toBe(200);
-  expect(afterRemove.body.quantity).toBe(0);
+  await waitForItemQuantity({ customerType: "user", customerId: userId, itemId, expected: 0 });
 });
 
 
@@ -796,7 +933,7 @@ it("does NOT auto-grant `free` when a non-internal tenancy's sub is canceled via
 
   const nowSec = Math.floor(Date.now() / 1000);
   const webhookResponse = await Payments.sendStripeWebhook({
-    id: "evt_customer_cancel",
+    id: uniqueEventId("customer_cancel"),
     type: "customer.subscription.deleted",
     account: accountId,
     data: {