Aus den FAQ: Q. What if a monster gets a Mate, or you get a Doppelganger, and then somebody backstabs you or plays potions on one side or the other? A. A Mate is an exact duplicate of the monster. A Doppleganger is an exact duplicate of the player. Anything that changes the level of one, changes the level of the other. (But if a monster is completely abolished - for instance, Pollymorphed - its Mate is unaffected because it's a separate monster.)