The rule is simple: - Within a transactional system you can have exactly once processing (as they call it here). - Crossing the boundaries between two transac

The rule is simple: - Within a transactional system you can have exactly once processing (as they call it here). - Crossing the boundaries between two transactions requires idempotency IDs to deduplicate redeliveries and restore exactly-once inside the new transaction. Email is an oft-cited example but is a transactional system that supports idempotency IDs (the Message-ID). Deliver the same email twice with the same message IDs and email systems are supposed to deduplicate. Gmail at least would do that, iirc.