[stickfigure]

It absolutely is idempotent behavior of the system. The goal is "make an idempotent payment", and this approach ("return an error for duplicates") was standard for financial APIs before Stripe. It still dominates for ecommerce APIs.

It isn't complicated, though I can see how if your entire experience with financial APIs is Stripe, you might not be aware of how simple it is. Because Stripe's approach, while mildly more convenient for clients, is a PITA to implement properly.

[whoamii]

You seem to believe idempotency is a fancy word for avoiding duplicates, which is why we seem to be having different conversations.

[stickfigure]

Idempotence is a fancy word for being able to repeat an operation without changing the outcome.

You seem to think that it's important to make the specific bytes of an http request-response idempotent.

I think that it's important to make a business operation idempotent.

You're missing the forest for the trees.