Is "character double" a term of the literary criticism, and what is the definition of "character double"?

"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, ....