Having a quick look through old notebooks, I found this fragment, the idea of which has stuck in my head since I wrote it down while living in Taipei around January 2007. I've edited it slightly.
A man was one day replaced. There's no arguing against a version of yourself, one that's willing to fill your place in life, especially when all your relatives are in support of the exchange.
For Arthur it was a case of leaving the room, when he never really needed to, only to find his double sat there when he came back to the room, in his very chair. This man not only had his exact physical features but he was wearing clothes not far off identical to Arthur's own. Still, this was a slightly slimmer model. A double with a better build. A double whose near identical clothes looked better fitting, not grubby. The double's hair looked how Arthur wore his on special occassions.
They observed each other, Arthur and his double, their heads tilted to one side. Neither had been warned about this eventuality but Arthur's double seemed to be taking it better than he was.