> You don't know, so you can't decide between retry and not retry

I'm well aware that the first order went through, even though the dumb system fumbled the translation of the success message and gave me a 500 back.

I do retry because I wanted the outcome. I'm not giving it a new key (firstly because I'm a user clicking a form, not choosing UUIDs for my shopping cart) but more importantly, if I did supply a second key, it's now my fault for ordering two copies.