Different Queue backends may provide different delivery guarantees.
|⩽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.