Idempotent Signup Emails in Node.js APIs

Short summary

Signup email duplicates usually result from misalignment between API writes, retries, and worker processing. Use a transactional outbox table to record email messages atomically with user creation, then let the worker claim rows exactly once using WHERE sent_at IS NULL. Test with isolated mailboxes per test run and assert message count matches the event key.

• •Signup email bugs stem from lack of coordination between API, retries, and notification workers on what 'sent once' means
• •Create an email_outbox table record atomically within the same transaction as user and token writes
• •Worker claims rows with idempotent WHERE sent_at IS NULL conditions to prevent duplicate sends on retry

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