I was thinking about a month ago, regarding LOST, "Oh, isn't it nice that if my favorite character dies, I have a back up?" And then my mind went on a rampage, and I realized that I was probably watching more sci-fi then is good for a girl. Because there is more then one. Of everything.
LOST. Best two-is-twice-as-fun episode: Recon.
Here there is a parallel universe, the same but a little bit different. It was introduced in season 6, with doubles of everyone. However it was a recently universe split, almost Terminator style. And as of now, no one has actually seen their double from the other world. They are living in blissful ignorance.
Smallville. Best two-is-twice-as-fun episode: Bizarro.
There is a clone, an evil clone, of Clark Kent, called Bizarro. Ah, lame comics, how I love thee. Their personalities are obviously different, but their appearances are exactly the same, thanks to stolen DNA and confusing Kryptonian science.
Doctor Who. Best two-is-twice-as-fun episode: Journey's End.
There was a biological split of The Doctor. One stayed with Rose and even if psychologically they were the same, they were biologically different. There is also a parallel universe in Doctor Who. However, no characters from both universes ever came head-to-head. Rose was a dog there, Pete was alive in the other world, but dead in our world, etc.
Fringe. Best Two-is-twice-as-fun episode: Over There.
One alternate universe but with more essential differences then LOST. Doubles of all the characters so far, except for Peter, only because his double is dead. Bonus: This show also has shapeshifters. However, the original must be dead for the shifter to take on it's form, therefore avoiding doubles.
The X-Files. Best Two-is-twice-as-fun episode: Small Potatoes.
There are no parallel universes, but the alien-hybrids can shapeshift. And unlike the Fringe shapeshifters, there can be multiple shifters walking around with the original. There are also Samantha clones, but how accurate a clone they actually are is unknown. Then there is that one episode where "muscle tissue" leads to Scully almost making out with Eddie Van Blundht.
Farscape. Best Two-is-twice-as-fun episode: Eat Me.
For half a season there are clones of John, D'Argo and Chi. They aren't really clones though, the original was split into two, making two perfect versions. D'Argo's and Chi's die early on, the two Johns have to duke it out and make do for about a season until one dies.
Terminator:tscc. Best Two-is-twice-as-fun episode: Allison from Palmdale.
There are multiple versions of certain models, like the Cameron model. Then there are the shapeshifter Terminators, which can take on any appearance, but like in Fringe, the original has to be dead first, before the Terminator can take on the appearance.
Okay, so I think somebody needs to slowly back away from the television. I just ate the last of the pecan rum birthday cake, covered in the last of the rum sauce. So until next year....