In some ways I'm tempted to use this device to explain some of the abhorent behavior of my mc - after serving time for a revenge murder and still not satisfied ( he's got a list for two more hits and is searching out the main bad guy ) goes off on the hunt. He starts off a nice guy , but with a lot of baggage , namely a twin who's a control freak , and is locked up in a mental ward. ( This may sound a little like Faster , but it's actually more film noir ) The twist was that as he goes off to start his second hunt , after his stint in jail , he switches places with the twin in the mental ward. 7 years in jail took a lot out of him and he no longer has the anger or ability to procede. As much as I like this, it takes some of the sting out of the initial idea of how far the original mc would go to exact revenge - and I could be cheating the reader by informing them , voila , it wasn't him, him all along.