>>attila+Deg
I agree, this whole thread seems to turn the concept of idempotency on its head. As far as I know, an idempotent operation is one that can be repeated without ill-effect rather than the opposite which is a process that will cause errors if executed repeatedly.
The article doesn't propose anything especially different from Lamport clocks. What this article suggests is a way to deal with non-idempotent message handlers.