The character double is a term in English that is often referred to by the German word "dopplegänger," "double walker," or "double goer." It is a common literary device found in folklore. It means that two characters, often physically so similar they might be twins, have very different personalities and serve as sharp contrasts or antagonists, called foils, in the themes or plot. The most basic opposition is good versus evil in their natures.
Two classic examples from American literature are Edgar Allan Poe's short story "William Wilson." The young William meets another boy at school who not only looks just like him, but even has the same name. The double always whispers, however. The story does not end well and includes a surprise revelation.
In a lighter vein, in The Prince and the Pauper, Mark Twain offers a tale set in sixteenth-century England of two boys so similar they might be twins. One, Tom Canty, is very poor; the other, Prince Edward, is the son of England's King Henry VIII. The plot involves them switching places.
The double concept is slightly different from the alter ego, which refers to two identities of the same person, such as Superman and Clark Kent or Dr. Jekyll and Mr. Hyde, but the terms are often used interchangeably.