React Mention Emails Without Double Sends

Short summary

Duplicate mention emails in React apps usually stem from optimistic UI updates and retry paths creating two mail jobs from one user action. The fix is a narrow server contract: the client submits one eventId with the mention list, the API stores it, and the worker refuses to reprocess the same eventId. This keeps React fast while ensuring email delivery stays idempotent, with simple tests that catch regressions immediately.

• •Duplicate mention emails arise from optimistic UI plus retry paths creating two mail jobs
• •Fix: client sends one eventId, API stores it, worker rejects duplicate eventId processing
• •Test with isolated mailboxes and assert exactly one email per eventId

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