Well, one of my favorite monsters from the old D20 Modern system was the 'Star Doppleganger', which was pretty much John Carpenter's version of "The Thing (from another world)" ^_^
It has a Lovecraftian feel, without truly being a Mythos entity (frankly, one of the things I didn't like about the 'Strange Aeons' adventure path was its very close association with the Cthulhu Mythos).
Of course it will take some work to convert it to Pathfinder stats, but not too much.
The hard part will be how to emulate it taking someone else's place, without the other player's knowing (easy to do for NPCs).
There was a Ravenloft adventure about Dopplegangers ("Hour of the Knife") dealt with PCs being replaced harshly: If a PC was ever in a situation where they were alone with a Doppleganger, they were automatically assumed to be killed and replaced (but this was originally a tournament adventure, so nobody was losing long standing characters themselves).