[hueyp]

http://queue.acm.org/detail.cfm?id=2187821

Section: "Zero or more times ... gauranteed"

"When considering the behavior of the underlying message transport, it is best to remember what is promised. Each message is guaranteed to be delivered zero or more times! That is a guarantee you can count on. There is a lovely probability spike showing that most messages are delivered one time.

If you don’t assume that the underlying transport may drop or repeat messages, then you will have latent bugs in your application. More interesting is the question of how much help the plumbing layered on top of the transport can give you. If the communicating applications run on top of plumbing that shares common abstractions for messaging, some help may exist. In most environments, the app must cope with this issue by itself."

At least once, at most once, etc are just probability distributions. All you can count on is zero or more times.