At-least-once delivery

Amazon SQS stores copies of your messages on multiple servers for redundancy and high availability. On rare occasions, one of the servers that stores a copy of a message might be unavailable when you receive or delete a message.

If this occurs, the copy of the message isn't deleted on the server that is unavailable, and you might get that message copy again when you receive messages. Design your applications to be idempotent (they should not be affected adversely when processing the same message more than once).