> Guaranteed delivery of a message once

That flies in the face of my distributed systems knowledge. It's not possible in some failure cases.

If your acknowledgement of a message gets lost (because either server involved or the pipes in-between fail) you've processed the message already but the queue server will think you haven't. It either has to resend it (duplicate delivery) or it ignores acknowledgements all together (drops messages that it sent you, but you didn't process - maybe because your server failed.) So the choice when there is a failure in the system is between at least once or at most once - exactly once cannot be guaranteed.

I'm not aware of any way around that predicament.