Dave (again) gave me a wonderful idea, only this one was from years ago when he was running his own unique game. Heavily influenced by anime themes, we and a bunch of the guys at Oakland University's Order of Leibowitz had a great time playing it because we were often encouraged to play outrageous characters. Mine, I thnk, was a bit more mundane than most; Ken Koda, super cop.
One night Dave takes me aside and says, "I want you to play a doppelganger that has replaced your character. Drop hints, give everyone else a chance to realize you aren't you." I had a ball all night doing evil shit, being a bastard, and yet no one caught on. It could have been that they simply were not expecting me to be not me, or I was not as out of character as I thought, or everyone thought I was being more of a dick than normal. It is hard to remember but I think Evil Ken led them into a trap.
There were no hard feelings, no outrage over the deception. My players seemed to have fun with the revelation. Keith especially had a great time. There were no hard feelings, either the first time years back or when I was running my game. If you pick the right player and if you play fair, giving the rest a chance to trip on to the switch, then the GM and players both could have a great time, provided there are opportunities for the characters to discover the deception.
The scheme truly works best if the campaign is a well established one with most of the players and their characters being familiar with one another. Perhaps to spice things up a hint is thrown that one of them is not who they appear, and perhaps there was an instance when everyone at one point or another was absent from the rest. Is there truly a double in their midst, or is the GM playing head games with the players?
This plot is not new. Marvel Comics had Secret Invasion where Skrulls had replaced numerous super heroes. In Deep Space Nine Doctor Bashir was replaced by a changeling. In The Original Series' Turnabout Intruder Kirk was forced to switch bodies with a woman named Edith Keeler. Picard was replaced at least twice on TNG and in Voyager I believe it happened to Tom Paris.
Which brings up a variation of this idea: body switching. What if everyone on the team/party wound up with someone else's body? They would have to learn one another's powers and perhaps deal with their civilian lives. Perhaps one character's original body is hideous and he doesn't want it back? Perhaps a person from one sex occupies the body of another and discovers they actually like being a chick/dude? There are all sorts of role playing opportunities here and a lot of potential fun.