[rmbyrro]

In this example, it seems the API should return some 4xx error.

If there's a process with two steps, moving from Step A to B requires the process to be at Step A and it's already at step B, it should return an error. At first glance, this doesn't seem the kind of problem that idempotency is supposed to prevent...

[wayfinder]

Instead of idempotency you can return a 4xx error too. You use idempotency because it’s logistically less complicated for the client.