How Idempotent Design Prevents Duplicate Signup Confirmation Emails in REST APIs

Duplicate signup confirmation emails are a common backend problem triggered when clients retry requests due to timeouts or network failures, causing multiple email intents to be created for a single signup attempt. The root cause is typically that APIs neither persist a stable request identity before queuing work nor give email workers a way to distinguish fresh events from replays. A recommended fix involves storing one database row per logical email intent — tied to an idempotency key — rather than one row per worker attempt, ensuring retries do not generate new logical work. An outbox pattern can then be used so workers only send emails when the persisted row is in a valid sendable state, with delivery timestamps recorded without altering the original intent. This approach gives support teams a clear audit trail, keeps frontend status models predictable, and makes signup email behavior easier to test and explain in production.