Dupes?

So there is this one guy on my friends list that can supposable dupe items for CS:GO. Here's the catch though, he can only dupe an item once and he does it in the steam client. My friend wants to trade him his knife but we don't know if it is legit. I tried to trade an item with a sticker or a name tag but he said he already duped the item. Is there anyway this guy is real or is he just buying it off the marketplace?

EDIT: He's offline right now but he has a M9 Bayonet Blue Steel in his inventory but I'm 99% sure he scammed it.