The odd thing about doppelgangers is all the work that has to be done to pull off a monster that makes itself an identical copy of a Player Character. They have to read minds to know all those secrets, they have to copy clothing, it can use magical items like the original, and so much more.

Of course, this never works because it takes the party ten seconds to ask the duplicate to cast a spell or drop their sword on the ground. It’s possible for a GM to trick a player, but an entire party is so much harder: it’s four against one.