Alex Byrne in the Boston Review:
Star Trek–style teleportation may one day become a reality. You step into the transporter, which instantly scans your body and brain, vaporizing them in the process. The information is transmitted to Mars, where it is used by the receiving station to reconstitute your body and brain exactly as they were on Earth. You then step out of the receiving station, slightly dizzy, but pleased to arrive on Mars in a few minutes, as opposed to the year it takes by old-fashioned spacecraft.
But wait. Do you really step out of the receiving station on Mars? Someone just like you steps out, someone who apparently remembers stepping into the transporter on Earth a few minutes before. But perhaps this person is merely your replica—a kind of clone or copy. That would not make this person you: in Las Vegas there is a replica of the Eiffel Tower, but the Eiffel Tower is in Paris, not in Las Vegas. If the Eiffel Tower were vaporized and a replica instantly erected in Las Vegas, the Eiffel Tower would not have been transported to Las Vegas. It would have ceased to exist. And if teleportation were like that, stepping into the transporter would essentially be a covert way of committing suicide. Troubled by these thoughts, you now realize that “you” have been commuting back and forth to Mars for years . . .
So which is it? You are preoccupied with a question about your survival: Do you survive teleportation to Mars? A lot hangs on the question, and it is not obvious how to answer it. Teleportation is just science fiction, of course; does the urgent fictional question have a counterpart in reality? Indeed it does: Do you, or could you, survive death?