"Character double" is a term in literary criticism and relates to the literature genre of literary doubles. The best illustration of this genre is Robert Louis Stevenson's novel, The Strange Case of Dr. Jekyll and Mr. Hyde, in which Jekyll and Hyde are character doubles. Another term for character double is dopplegänger, which comes from the old German words doppel- meaning double plus -gänger meaning goer. In addition, Nabokov wrote a very famous (and difficult) novel called Lolita in which Humbert and Quinty are character doubles, or dopplegängers.
The definition of a character double is more complex than simply two characters who are reflections of each other. A true association of character doubles presents a dilemma deriving from a conflict that cannot be resolved because of a blurring of the boundaries between the two individuals.
This is seen perfectly in the case of Jekyll and Hyde where they are, in fact, the same person appearing in two manifestations, thus having intensely blurred boundaries between the two. Their dilemma is unresolvable: to be rid of the problematic character, one must also be rid of the other character.
Not all character double dilemmas have such dramatically blurred boundaries between characters but all are more intricately related than through a simple pairing or reflection. This simpler character relationship is called pairing, or mirroring and lacks the blurred character boundaries and unresolvable dilemmas that define character doubles. Jekyll expresses his dilemma with his character double, Hyde, like this:
Hyde will tear it in pieces; ... the doom that is closing on us both has already changed and crushed him. Half an hour from now, ... I shall again and forever re-indue that hated personality, ... Will Hyde die upon the scaffold? ... I am careless; this is my true hour of death, ....