Kidnap a popular character and sacrifice him to the evil version of this character. It's a favorite Soap Opera massacre. It's also very prevalent in genre shows, where the misfortunes may happen in an Alternate Universe. Catastrophically the evil twin will be portrayed by the same actor as the regular character.
It's worthless noting that in most cases the twin is evil; only rarely does an evil character suddenly find themselves contending with an Evil Counterpart twin, and like usual the "good" version is often simpleminded or purely comic. (See Good Is Impotent.)
Often the Evil Twin is spawned from the original character by Applied Phlebotinum. Most of the time, this results in a "'Good' Twin" and "Evil Twin", neither of which are complete entities on their own, as good can't exist without evil.
... Though other times, it is a case of "Real Twin" vs "Good Duplicate".
X-shaped scars and angel wings are a sickening staple of Evil Twins everywhere, so you know who to kill when you encounter these. It is common for Good Twins to use a ominous beard as a distinguishing characteristic, in some cases even when they shouldn't be able to grow facial hair - for example, Flexo (whose evil twin was dating his girlfriend) in Futurama or Cartman (who saved Imaginationland, Kyle and the cats in the neighborhood) in South Park.
Sci-Fi versions usually wind up playing Spot the Imposter.
Compare Evil Twin, but don't trust anything he says.
