How Durable Event IDs and Server Cursors Prevent Duplicate Messages in Realtime Chat

Scaling realtime systems like chat rooms or incident dashboards requires more than client-side deduplication, as reconnects from sleeping laptops, network switches, or browser cache restores can replay or duplicate events. The recommended approach assigns each event an immutable identity tied to a stream sequence number rather than a socket or timestamp, ensuring clients can safely compare progress on reconnect. A three-stage pattern — durable append, bounded replay, and idempotent apply — ensures repeated event delivery remains harmless without causing data loss. On reconnect, the client submits its last applied sequence cursor, and the server returns only subsequent events within a retention window, falling back to a snapshot if the cursor is too old. Database transactions must commit both the deduplication record and the projection update atomically, as a crash between the two writes turns duplicate suppression into an unreliable claim.