Yeah, but that just seems like a bizarre definition on which to base the claim that you cannot have exactly-once delivery. Obviously, if you define delivery to preclude de-duplication, then you can't have exactly-once delivery. But you can have something that delivers messages (for some reasonable definition of "delivers") exactly once. It seems weird to define delivery in such a way that such a system does not provide exactly-once delivery.
I'm not sure how to convince you to get over your hangup around how other people feel about the word "delivery", but this is exactly why others distinguish between "delivery" and "processing". If you think there are better terms than those two to describe "the system that receives a message and must be responsible for de-duplication" and "the system that can rely on messages being already de-duplicated", then feel free to propose them and have people debate, that I suppose. But because of what I noted earlier, this is a very useful distinction to maintain for people working on systems that are responsible for de-duplication (likely most people on this forum), and these words seem to make sense for most individuals.
I don't have any hangups about how other people feel about anything. My "hangup" is that I see no evidence that there is a consensus on a technical definition of the word "delivery". In the absence of such a definition, there is no basis for asserting that exactly-once delivery is impossible, particularly when there are existence proofs to the contrary based on reasonable informal definitions of the word "delivery".
