Quote Originally Posted by The Giant
There's a rule to handle it, though I forget where in the rulebook. Redcloak is Unique, so when you play him, the previous version of him is removed from the board.
Page 5, in the section that addresses monster name:

"The monster name is mainly for flavor. However, some cards are designated Unique (green name and border). If there is another Monster with the same name already in play somewhere in the dungeon, then that Monster goes to the discard pile when this card is added to a battle."