The impossibility of "exactly once" is a theorem, not an opinion.
Knowing that, the article you linked is funny. It begins with maybe a thousand filler words complaining that all of this is "poorly understood", and it's not "impossible", just "very hard". Then it gets to the meat: Yeah well, you know, it's not quite "exactly once delivery", just "exactly once semantics", and to achieve that, messages need to be idempotent so that duplicates don't matter.
> The impossibility of "exactly once" is a theorem, not an opinion.
It's quite likely that your definition of what "exactly once" means differs from the one followed by MQTT. As this issue was documented years ago, I doubt this is a relevant argument to have, unless we want to feel smart by criticising others.
That's the Kafka Streams API. "Exactly-once semantics" has a very specific meaning in the context of that particular API, which the article could probably do a better job of clarifying upfront. (Otherwise it is an excellent overview of the problem and solution provided by the Streams API.)
Do you mean like Confluent do? https://www.confluent.io/blog/exactly-once-semantics-are-pos...