SQS messages processed once, not three times.

The compliance trigger was the worst kind: an identity verification was charged twice on the customer because the underlying SQS message had been processed twice and the system wasn’t idempotent. The underlying cause was a routine Lambda slow path crossing the 30-second visibility timeout.

We set the visibility timeout to six times the Lambda’s p99 duration (a common rule of thumb) and added idempotency via DynamoDB conditional writes keyed on the message ID. The DLQ retry policy stepped from "fail after three" to "fail after five" — most of the previously-failed-after-three messages succeeded on the fourth attempt.

Duplicate processing dropped 99% — the rare remaining cases trigger the idempotency check and short-circuit. Compliance cleared the KYC duplicate-charge finding. The team has the same Lambda code, just better queue semantics around it.