Which is what you -do- to handle 'at least once' delivery. The system can try and hide that complexity from you,but there are still tradeoffs in any implementation. How long in between does that guarantee hold? Does it guarantee that if you fire the same message a year from now it will still recall that it's a dupe (i.e., persist all message identifiers for an infinite amount of time)? Probably not. Does that matter to you? Maybe!

Even if it does persist, does it persist the identifier on receipt of the message, or on sending it to you for processing? If the former you run the risk of crash and never having handled it; you really have no guarantee of delivery to your processor. If the latter, what happens if the receiver crashes before it hands it off to be processed? If simultaneous, is 'processing' atomic? Probably not; what happens if you crash midway through processing the thing? Etc.

That's my point; you need more details to make the system robust. You don't get "exactly once delivery" out of the box; you get a system that attempts it by deduplicating, but there be gremlins, and the fact you're not saying "it's at least once delivery with (details)" means I'm not hearing a technical pitch, but a marketing one.