Receiver is processing the same event twice

This page covers what “receiver is processing the same event twice” usually means and how to recover.

Symptoms

• Same order_id billed twice.
• Two confirmation emails for one action.

What usually causes it

• Harbor retried the delivery because the receiver’s 2xx was delayed and Harbor timed out.
• Receiver acked before persisting — the first ack succeeded from Harbor’s side but the work was lost.
• Receiver doesn’t check event_id before acting.

How to fix

• Key your side-effects on event_id. Insert-if-not-exists before doing the expensive work.
• Ack fast (sub-second 2xx), then do work async — but only if you durably enqueue the work before ack’ing.
• Raise your Harbor timeout config if your processing is fundamentally slow (not recommended; prefer async).