How to Prevent Duplicate Signup Emails in Node.js Using an Outbox Pattern

Duplicate signup emails in backend systems are often caused not by SMTP failures but by misalignment between account creation, retries, and background job workers. The core issue arises when the HTTP layer or worker retries after a partial failure, sending multiple welcome or verification emails even though each subsystem appears healthy individually. A reliable fix involves writing a dedicated outbox record within the same database transaction as the user and token creation, ensuring atomicity across all steps. A mail worker then claims each outbox record exactly once using a conditional update that checks for a null sent_at value, preventing re-sends on retry. This transactional outbox pattern, enforced via PostgreSQL's conflict handling and row-level locking, keeps the API responsive while making the notification pipeline safely idempotent.