As a maintainer of logging middleware (https://www.fluentd.org) I am asked all the time whether we have "exactly once" semantics. I try to be honest and say we err on the side of "at least once" message delivery, but even then, there are edge cases where messages can be lost.

It really vexes me to see how a lot of distributed systems projects claim to support "exactly once", and it makes me happy to see a grounded, intelligent summary like this one.