What is the difference between guaranteed message delivery when a message is guaranteed to be delivered no more than once and when it is guaranteed to be delivered at least once?

The "at most once" message delivery guarantee means that a message can be delivered either once or not at all — duplicates are excluded, but message loss is possible. This is important when duplication is unacceptable.

The "at least once" guarantee means that a message will be delivered at least once, but duplicates may occur if delivery confirmation is not received and the message is resent.

Example:

• At most once: the system sends the message and does not resend, even if confirmation is not received.
• At least once: the system resends until confirmation is received, which may lead to duplication.