honestly, there would be an easy way to get rid of dupe all togeter.
Give every item their own unique code and the ability to show how many items have the same code. so for example even if two people get the same item from a gacha, the only difference would be their code which is different and on the item each would say "this code is only present once"
If someone dupe an item, the duped item would end up with the same code as the original and both item would say "This code is present 2 times"
A duper will always have the original item they duped which will get flagged as a Duped item too and Nexon could just peridiocaly delete all items that have the same code as another item.
When trading and buying, you would be able to read the mention if this item is unique or duped so if you dont want the item to be deleted by nexon, dont buy those that say are duped.
Not only would this clear up the issue with people duping and selling dupes, they would actually get punished by having their original deleted as well.