Buyer Entered another code

The buyer activated the early code with gold loot box from the overwatch and bought from me the same product. The buyer can’t enter the code and filed a complaint about the refund. What should I do in this situation? The code is still working

Hey If you specified within the listing description that the item could only be redeemed once per account, please contact our support team and tell them about this situation.

The dispute will be probably decided in your favor after investigation.