For dynamic effect, I will present this part of the plot as If I were running an adventure and you are a fighter. However, keep in mind that the setting and the player can be as diverse as you want.
“With the knowledge that the evil mage has been defeated, you decide to separate from you’re party and see what kind of treasures await you in the various rooms of his castle. You walk down a dimly lit corrider and into the room ahead of you. The room has a dry parched smell about it and there is various furniture in the room.”
“As you take a step inside, the torch is knocked out of you’re hands by a fighter and the face of you’re assailant is lost to the stuggle at hand. The fighter is every bit as capable as you are, and returns each blow for blow.”
“After a tense struggle for you’re life, the fighter runs out of the room and through the back door of the house. You are badly wounded, sporting gashes and cuts all over from the tense struggle, and realizing that healing is more important than catching this dark figure, you return to you’re party for healing.”
“You inform the rest of the party of the fighter in the other room and you decide to stay with the party while you go room to room searching for treasure just in case he returns.”
“A little later, you are in a well lit room with the rest of your party, searching a barrel in the northern corner of the room for anything that you might find of value. As you are searching the barrel, you burst in through the door to the south and see an imposter of yourself looking through a barrel in the northern corner of the room. What do you do?”
(you might want to read that last part again, just in case you were caught off gaurd the first time)
OK, so, behind the scenes, what really happened is the fighter walked alone into a room with a demon mirror, a legendary artifact that creates a false duplicate of a person, complete with equiptment. The mirror demon takes an adventurer’s exact form, complete with items, equiptment, knowledge, skills, abilities, and anything else that defines that character, but the demon is also evil.
There is a struggle, and depending on how the fight goes either the adventurer or the demon wins, returning to the party as that player.
The real beauty here, is that you always make the player think that he/she won the fight and that the unseen figure ran away (although he could be playing either the person or the demon). Later when the player has returned to the party, you tell the player that while he is doing something completely unrelated, that he enters the room to see an imposter of himself in the room with the party.
Understand that while the demon knows who he is, that he is also going to act exactly like the real player in order to prevent discovery. Only when nobody is watching would he do something uncharacteristic of his copy. In other words, players would only see the effects of demon exercising his free will, but they would never catch him on their own.
This is perfect for two reasons.
1. There is no metagaming. Since nobody actually knows who is who, then it isn’t possible for characters to metagame and unfairly discover the identity of the demon.
2. Since even the player himself is not aware of who is the real him, and who is the imposter, there is an actual suspense created by the real life mystery of who is who, something that can be hard to do sometimes.
If both copies of the player are allowed freedom, then during the night, strange things start to happen. First somebody is missing a magical pendant. Then somebody else finds the water in their canteen poisoned ect. The players know that one of the two is sabotaging the group, but they don’t know who.
On one hand, if they let both act freely, bad things will keep happening. On the other hand, if they tie them both up, they are down not one player, but two.
(Now I have given you the main Idea, I will talk about how to handle the situation as a whole and keeping everybody happy)
Now, I think that it is important here to actually use the player reactions govern the characters that they act out. So in other words, when a player goes, “HUH?” have the character do so as well.
Sometimes a player will have trouble with the concept of roleplaying such a thing. By sometimes, I really mean always. In order to avoid the problem, it is necessary to keep presenting the player with the scenario again and again from his new perspective.
(For example,: the player might say that he wants to draw his sword. I say, “You hear somebody behind you draw a sword. As you turn around, you see an imposter of yourself wielding a weapon at you. You recognize this as the person who ran off earlier, but he seems healed now. You realize that while you may have won before, that it was just luck that you were the victor and not he. What do you want to do”
It is espescially important that you re-discribe the actions that the “Imposter” took, and keep asking the player what he would like to do in response.
Meanwhile, the other players can act as themselves. You would tell them that there are two duplicates of the same character, and that while one of them must be an imposter, that it is immediately impossible to tell which one is which.
I like to keep it up for a while. Let players really react to things. Then I give them a quest to find out the identity, usually having them run a mission for a high level cleric, for the exchange of having the true identity detected. It should be noted that the players themselves should not be of high enough level to discern one from the other without help, but you can call it off whenever.
A good Idea is to present the entrance of the second duplicate as the last thing that happens in the adventure. Sort of a cliffhanger for everybody when the second fighter comes in and says “He is an imposter”.
Then you can talk to the individual player at a little more length about what has happened (without telling them which character is which), and give them some time to cope with the idea so that you don’t spend the entire adventure time putting out the absolutely magnificant forest fire that you just created.
*This was edited after talking a little to Mourngrym*