There's a place for idempotency tokens. They're relatively easy to retrofit onto old systems, and occasionally they are the best way to go about making changes

There's a place for idempotency tokens. They're relatively easy to retrofit onto old systems, and occasionally they are the best way to go about making changes idempotent, but they should be a red flag – an indication that you should step back and see if maybe you can redesign an API to make idempotency a natural guarantee rather than something you artificially strap on with a token. As a rule of thumb, I would always mention the idea of adding an idempotency token, and prompt for alternatives, with all stakeholders present. (In this particular example, I agree that an idempotency token is probably the way to go, as otherwise callers would need to somehow distinguish between their callers trying to create a duplicate account, versus something going wrong with them resulting in duplicate requests. I just have too often seen developers conflate "idempotency" with "use an idempotency token)