diff --git a/.server-changes/prevent-duplicate-root-environments.md b/.server-changes/prevent-duplicate-root-environments.md
new file mode 100644
index 0000000000..0710595deb
--- /dev/null
+++ b/.server-changes/prevent-duplicate-root-environments.md
@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Prevent duplicate Staging and Preview environments when account setup requests overlap
diff --git a/apps/webapp/app/routes/admin.api.v1.orgs.$organizationId.environments.staging.ts b/apps/webapp/app/routes/admin.api.v1.orgs.$organizationId.environments.staging.ts
index f3c215bfd4..cf6d1abce0 100644
--- a/apps/webapp/app/routes/admin.api.v1.orgs.$organizationId.environments.staging.ts
+++ b/apps/webapp/app/routes/admin.api.v1.orgs.$organizationId.environments.staging.ts
@@ -1,5 +1,6 @@
 import { type ActionFunctionArgs, json } from "@remix-run/server-runtime";
 import {
+  Prisma,
   type RuntimeEnvironment,
   type Organization,
   type Project,
@@ -61,9 +62,16 @@ async function upsertEnvironment(
   type: RuntimeEnvironmentType,
   isBranchableEnvironment: boolean
 ) {
-  const existingEnvironment = project.environments.find((env) => env.type === type);
+  const existingEnvironment = project.environments.find(
+    (env) => env.type === type && env.parentEnvironmentId === null
+  );
 
-  if (!existingEnvironment) {
+  if (existingEnvironment) {
+    await updateEnvConcurrencyLimits({ ...existingEnvironment, organization, project });
+    return { status: "updated", environment: existingEnvironment };
+  }
+
+  try {
     const newEnvironment = await createEnvironment({
       organization,
       project,
@@ -72,8 +80,23 @@ async function upsertEnvironment(
     });
     await updateEnvConcurrencyLimits({ ...newEnvironment, organization, project });
     return { status: "created", environment: newEnvironment };
-  } else {
-    await updateEnvConcurrencyLimits({ ...existingEnvironment, organization, project });
-    return { status: "updated", environment: existingEnvironment };
+  } catch (error) {
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === "P2002") {
+      const existingAfterConflict = await prisma.runtimeEnvironment.findFirst({
+        where: {
+          organizationId: organization.id,
+          projectId: project.id,
+          type,
+          parentEnvironmentId: null,
+        },
+      });
+
+      if (existingAfterConflict) {
+        await updateEnvConcurrencyLimits({ ...existingAfterConflict, organization, project });
+        return { status: "updated", environment: existingAfterConflict };
+      }
+    }
+
+    throw error;
   }
 }
diff --git a/internal-packages/database/prisma/migrations/20260714140000_runtime_environment_unique_staging_preview_root/migration.sql b/internal-packages/database/prisma/migrations/20260714140000_runtime_environment_unique_staging_preview_root/migration.sql
new file mode 100644
index 0000000000..153db8c2d0
--- /dev/null
+++ b/internal-packages/database/prisma/migrations/20260714140000_runtime_environment_unique_staging_preview_root/migration.sql
@@ -0,0 +1,4 @@
+CREATE UNIQUE INDEX CONCURRENTLY IF NOT EXISTS "RuntimeEnvironment_projectId_type_staging_preview_root_key"
+ON "RuntimeEnvironment" ("projectId", "type")
+WHERE "parentEnvironmentId" IS NULL
+  AND "type" IN ('STAGING', 'PREVIEW');
diff --git a/internal-packages/database/prisma/schema.prisma b/internal-packages/database/prisma/schema.prisma
index 1b8642e866..2800dd5dd2 100644
--- a/internal-packages/database/prisma/schema.prisma
+++ b/internal-packages/database/prisma/schema.prisma
@@ -394,6 +394,8 @@ model RuntimeEnvironment {
   taskIdentifiers         TaskIdentifier[]
   revokedApiKeys          RevokedApiKey[]
 
+  // A partial unique index also enforces one STAGING/PREVIEW root per project and type.
+  // It is defined in SQL because Prisma does not support partial indexes in the schema.
   @@unique([projectId, slug, orgMemberId])
   @@unique([projectId, shortcode])
   @@index([parentEnvironmentId])