diff --git a/apps/dashboard/src/app/(main)/(protected)/projects/[projectId]/users/page-client.tsx b/apps/dashboard/src/app/(main)/(protected)/projects/[projectId]/users/page-client.tsx
index c2f46cb40..2f9852a5f 100644
--- a/apps/dashboard/src/app/(main)/(protected)/projects/[projectId]/users/page-client.tsx
+++ b/apps/dashboard/src/app/(main)/(protected)/projects/[projectId]/users/page-client.tsx
@@ -10,6 +10,7 @@ import { PageLayout } from "../page-layout";
 import { useAdminApp } from "../use-admin-app";
 
 function CreateDialog(props: {
+  existingEmails: string[],
   open?: boolean,
   onOpenChange?: (open: boolean) => void,
   trigger?: React.ReactNode,
@@ -17,7 +18,7 @@ function CreateDialog(props: {
   const adminApp = useAdminApp();
   const formSchema = yup.object({
     displayName: yup.string().optional(),
-    primaryEmail: yup.string().email().required(),
+    primaryEmail: yup.string().email().notOneOf(props.existingEmails, "Email already exists").required(),
     primaryEmailVerified: yup.boolean().optional(),
     password: yup.string().required(),
   });
@@ -56,7 +57,13 @@ export default function PageClient() {
   const allUsers = stackAdminApp.useUsers();
 
   return (
-    <PageLayout title="Users" actions={<CreateDialog trigger={<Button>Create User</Button>} />}>
+    <PageLayout
+      title="Users"
+      actions={<CreateDialog
+        trigger={<Button>Create User</Button>}
+        existingEmails={allUsers.map(u => u.primaryEmail).filter(e => e !== null) as string[]}
+      />}
+    >
       {allUsers.length > 0 ? null : (
         <Alert variant='success'>
           Congratulations on starting your project! Check the <StyledLink href="https://docs.stack-auth.com">documentation</StyledLink> to add your first users.