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.