Delivery Guarantees

Different Queue backends may provide different delivery guarantees.

Code	Name	Definition
⩽1	At most Once	Messages will not be duplicated, but may be lost.
=1	Exactly Once	Messages will be neither lost nor duplicated. You will receive every distinct message exactly once.
⩾1	At Least Once	Messages will not be lost, but can be duplicated.

This StackOverflow question has a few good answers explaining these concepts.