What is a good practice to achieve the "Exactly-once delivery" behavior with Amazon SQS?

FIFO queues are now available and provide ordered, exactly once out of the box.

https://aws.amazon.com/sqs/faqs/#fifo-queues

Check your region for availability.

The best solution really depends on exactly how critical it is that you not perform the action suggested in the message more than once. For some actions such as deleting a file or resizing an image it doesn't really matter if it happens twice, so it is fine to do nothing. When it is more critical to not do the work a second time I use an identifier for each message (generated by the sender) and the receiver tracks dups by marking the ids as seen in memchachd. Fine for many things, but probably not if life or money depends on it, especially if there a multiple consumers.

Conditional writes sound like a clever solution, but it has me wondering if perhaps AWS isn't such a great solution for your problem if you need a bullet proof exactly-once solution.