[Smrchy]

It's not exactly "once". It's exactly "one" within a defined period of time. So in that articles context it would be the "at-least-once" variant.

If the recipient fails to process the message for some reason it will reappear after the visibility timeout.

[zwily]

I don't think you can provide exactly once even with that constraint. What if the defined period of time was 100 years?

[kasey_junk]

Then you need a very big buffer.