I also encountered the same issue when using StoreKit 2, and I'm not sure under what circumstances this 'unknown' error will be thrown. I don't know how to explain it to our app's users.
IamJerry
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I encounter the same problem when using StoreKit 2. duplicate charges in one purchase. two transactions with the same appAccountToken but different original transactionId.(a non-renewing subscription product). the user have already given us feedback twice. the phone runs iOS 18.0.1