Update: I kept refreshing the page and magically managed to get through a confirmation screen. I clicked on purchase hoping that it would confirm and it gave me the same error message. Got this in the console:
agaS95
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Have you ever managed to get to the bottom of this? Currently trying to figure out the same thing. I've found a way around it by simply using GKPlayer.displayName (as I understand they're also meant to be unique) but this solution is far from ideal.