The cases that matter are the ones a replay cache does not explain:

• completed replay

• concurrent retry

• partial local success

• downstream unknown state

• same key with a different canonical command

• duplicate operation without a key

• retry after expiry

• retry after deploy, schema change, service hop, or region failover

If your design only handles completed same-command retries, it is a replay cache. That might be enough for some endpoints. But it is not the whole problem.