Currently, there are 2 cards that can do this: Yoren and Euron
Reaction: After you marshal Yoren, choose a character with printed cost 3 or lower in an opponent’s discard pile, and put it into play under your control.
Reaction: After Euron Crow’s Eye discards a card using pillage, put a location from the losing opponent’s discard pile into play under your control.
My question is about how this interacts with the rules about unique cards.
1) Does a copy of that card in your dead pile prevent you from putting that card into play? Obviously yes.
2) Does a copy of that card you own or control in play? I'd say yes, because you're taking control of the card (Ownership and Control, p14, says players control cards in their out-play areas) and you're not allowed to do that if you own or control a copy of the card in play.
3) Does a copy of that card in its owner's dead pile? I'd say no, because that case is not covered by the rules about unique cards.
4) Does a copy of that card in play owned or controlled by the player you're taking it from? I'd say no as well, for the same reason.
I don't like the answers to 3 and 4, because they seem to go against the spirit of the rule.
And as a bonus question, assuming my answer to 4 is correct, would the opponent in question be allowed to dupe their card afterward (assuming both the copy under their control and the one under mine are still both in play)?