A character in a story has been replaced, and no one notices. At least, not at first. They don't, because the replacement is a copy. Or at least a look-alike. It could be anything from a coincidental look-alike through an intentional clone to an inhabitant of an alternate dimension. It's something of a Sci-Fi cliché, mostly because the means of duplication are more varied in fantasy and science fiction, although more main-stream dramas and comedies have also used an Evil Twin theme.
But whatever the mechanism, there is a certain formulaic predictability to the Evil Twin story. From a writer's point of view, an Evil Twin episode is terribly easy to plot out. You need a mechanism by which a character is copied, a means by which the other characters figure it out and fix it, and a confrontation. As time allows, the duplication can be used as either a concerted attack on the characters or their organization, or just a series of errors of identification and assumptions.
The mechanism depends on the limitations of the genre. The confrontation is either of two standards. Good versus evil while Security tries to decide who to shoot. Or Real and Copy show up together by accident, probably ruining an attempt to pass one off as the other, and everyone is confused until Security decides to shoot them both.
So, since it is so common, and so predictable, I expect a great deal of resistance to my idea about Evil Twin episodes.

It is the single greatest theme in Science Fiction.

Yes, I know, I just pointed out what clichés they are. But bear with me. I'm talking theme, not plot.
Science Fiction is at its most powerful when it is used to examine the human condition. To look at our society, at ourselves, from the special perspectives that the platform offers.
"Fraser" tends to look at the pompous of society from the point of view of the middle class. The character of Fraser is charged with defending the superiority, or at least the preference, of his eclectic decor over the armchair of his father. Or gourmet over steak and potatoes.
With a Sci-Fi show, one can explore more basic parts of human existence. "Never mind eclecticism, why decorate your cave with pictures of trees? Why not live where trees are?"

We cannot know ourselves until we can step outside of ourselves. And an intelligent cloud's view of humanity is fairly far outside. Or at least it should be. A lot of 'SF' aliens are like Greek Deities - humans writ large or only slightly skewed. Still, they allow us to concentrate on an aspect of human nature. The cultures Gulliver traveled through in Swift's novel are great examples. Gulliver goes from being an observer, commenting on the parody of Swift's view of English society, to being the insider, defending his society to the views of critics.

So to me, the crux of the Evil Twin episode is that the replacement must always be discovered. Some characteristic of the replacement keys the co-workers or family to the fact. In some cases, it's just not a very good copy. In a Stargate episode, Jack was copied, but the clone was not the right age. By a factor of decades.
In other examples, smaller physical changes reveal the ruse, such as missing scars, an ability to look directly at the sun, big fangs, being bulletproof or having silvery fluid for blood. These are rather indisputable, once they're noticed.
For my money, the best indicators are personal character traits or habits, things that aren't apparent to a casual acquaintance, but only to close friends. Things like holes in memory, use of racist remarks, competence in the use of alien technology, kissing wrong, attempting to seduce subordinates, singing drinking songs on the mess decks, using contractions and other subtle clues that build up suspicion.
This is one of the strongest uses of the plot device. Not just questioning 'what is humanity' but 'what is ME?' What makes ME unique, even if surrounded by clones? How can I prove myself to be more real than the copy?
If the answer is 'I don't have metal parts' that's cheesy.
If the answer is 'because only I can connect on an emotional level with my soul mate,' that's romantically cheesy, but at least it's a step above checking for batteries.
The best answer is 'because only I am really human, only I am obviously ME.'

To me, the questions sparked by an Evil Twin theme (Why should my crewmates pick me over the other guy? If I were replaced by a robot, how would anyone ever know?) show that the theme is related to the Possessed/Brainwashed theme. It is the correct body, but the actions taken show alien (in the 'external' sense of the word, not necessarily LGMs or BEMs) influence, and the plot usually works out the same, minus the Good/Evil twin face-off confrontation in front of the confused of witnesses. Well, not minus, but replaced by the 'Fight for control of your own mind/body' scene.

So, I agree that it's hopelessly clichéd, and overused, and all too often poorly implemented. But I maintain that it has some of the greatest potential in SF.



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