Some might call this pedantic, but I think physical reality provided us the earliest idempotency keys, which were natural keys in specific systems. Some effects are idempotent because they address physical reality in a way that converges or provides mutual exclusion.

The pigeon-hole principle isn't an instruction guide for birds, it just describes the reality of occupancy. An egg-tooth keeps pecking, escaping at most one egg. A dog may circle, but occupies at most one bed.

The real value of idempotence is being truly end-to-end. It is about the final effect being controlled, rather than about some message hop in the middle. In my opinion, the impulse to isolate it to a messaging layer is often misguided. It presupposes that the other layers don't also have similar failure modes.

It makes for a more robust system when you can elicit the desired idempotence from natural keys and occupancy rules. For example, the way REST can make creation idempotent when the name can be established ahead of the creation request, or deletion is idempotent when names are not reused.