Q: What is it like to be a twin?
Which brings us to tonight's real topic: characters in science fiction meeting themselves in alternate universes. Generally alternate universes use the good twin/bad twin, Cain/Abel model, so the good guys have to defeat the bad guys, who look just like them. On Fringe in season 3, one of the main characters met her double, and they instantly fought. A long time ago, on Stargate SG-1, the woman on the team met her double, and they were standoffish with each other until the end of the episode, when one of them allowed as how the other's hair could actually be cute.
As a twin, I am fascinated by these episodes because I can't understand why people can't cooperate with their doubles, plus, the first, not the last, thing that you do is admire each other's hair because you just got a free demonstration of something that would be flattering on you. (Also check for cute clothes and shoes.) You don't always have the same goals as your sister, but you can always understand what's going on and work something out.
The only show I have ever seen that handled the double episode properly was Buffy the Vampire Slayer, where in one episode Xander got cloned. After they concluded that the extra Xander wasn't a monster or a robot, the two Xanders became instant best buddies and were hilarious together. The two Xanders were played by the regular actor and his twin brother. Maybe they gave the writers an idea...?