Acing a copy of a unique card that you have in play
Nothing special happens and the unique character in play is permitted to remain in play, despite a duplicate being added to Boot Hill. The uniqueness rule only applies to bringing cards into play, not to cards that are already in play.


  You can put more than one copy in your deck, but you can’t bring a unique card into play if you own another card with the same name that is either already in play or in your Boot Hill. (DTR Rulebook, Page 14 Uniqueness)


This is corroborated by a ruling on the AEG forums.