Short answer: if a shipment reminder can be delivered twice, assume at-least-once queue processing and claim a durable idempotency key before sending; queue-level deduplication is a useful filter, but its 5-minute window is not the correctness boundary. For a media shop sending a shipment update to many subscribers, I would choose durable send keys over deduplication alone. Ack only after the update has been safely persisted or sent. The catch is operational: the send-key store becomes part o...