Comment by mlyle

To discard the duplicates, you need to update your state based on the first message and consult that state for all subsequent messages. Someone must be delivered those messages for this to happen.

If you say that someone can then pass you messages exactly once over a perfectly reliable channel, well.. sure. But the system still has to deal with duplicate delivery and update its state based on that. (And potentially must durably persist this state. And, then, if your application doesn't have exactly the same persistence/transactional boundaries as what's doing the deduping, you have problems...).