> That way you can send as many retries as you like, as long as they’ve all got the same idempotency key - the operation will only be performed once.

I worked in an org where idempotency meant: if it threw an exception this time, it needs to throw the same exception everytime.

Pragmatism rules here, but yeah - the common way to do this (at least if you have keys generatable by the client), eg. using REST, is to not allow POSTs, but only PUT. Most APIs I've seen use PUT solely for updates (of existing items), but as is obvious from the wording it's not the original intention.