What would you do?

Write a ticket reporting that the item you bought unknowningly was replaced with an exact duplicate which makes you suspicious it's a duplicated item. I'd use it until they either close the ticket or give you further instruction, they would probably remove it and possibly reimburse you maple points because of it.

Same as this, but they'd probably just close it with an auto reply saying don't buy things you suspect are duplicated in the future (which you obviously didn't because you didn't realise 'til after you got it, but Nexon).