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.

Normally I'd say that this is the most ethical and reasonable course of action - but haven't there been people who've gotten punished (with suspensions or something) for reporting stuff like this because Nexon CS has zero reading comprehension? I don't know if it's true or not, but I wouldn't put it above CS to do this sort of pomegranate.

IMO, a safer route would be to get rid of the item first (by NPCing it) and then report it anonymously.