[vp8989]

"Exactly-once semantics is virtually impossible to achieve".

Impossible to achieve for a broker, yes. Combination of at least once delivery everywhere upstream and deduping in the consumer gives that consumer exactly once processing.

Lots of people don't grok that and they can be duped into buying certain products with claims that they offer "exactly once". Kafka does or used to do this, SQS (FIFO) also gives this false impression although they are a little more subtle about it. As soon as 1 vendor does it, they all have to do it.

"Did you hear Rabbit doesn't dedupe messages?!"