No, that's not what the question was. The parent poster specifically said "you don’t know if they kept a copy of the key. But is there some mechanism whereby it

No, that's not what the question was. The parent poster specifically said "you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?" In your example the account key on the usb stick is not guaranteed to be the only copy, and thus, there is risk of the sender double spending that money.