Doubling in literature occurs when a pair of characters can be described as two sides of the same coin, though one represents the evil or simply hedonistic side of the other. This concept may have originated in mythology, but it rose to popularity in Gothic literature.
While the idea of the "evil twin" dates back to ancient times, the most well-known examples of doubling are found in Victorian Gothic novels. Jekyll and Hyde, Van Helsing and Dracula, and Dr. Frankenstein and his monster are famous examples of symbolic doubling. It is also found in the story "William Wilson" by Edgar Allen Poe. One character represents either goodness or rational, restrained thought, whereas the other represents the shadowy other.
Doubling is most often associated with the dark, psychological tales of the Victorian era, but the modern-day double, or Doppleganger, character appears in works ranging from Hitchcock films to soap operas. The movie "Fight Club" is a prime example of doubling, in which Tyler Durden represents the main character's repressed hedonism and violence. In stories revolving around a double, the ending usually involves a confrontation between the two characters, which is really the main character confronting his conflicted soul. Doubling remains a popular way for writers to externally represent a character's internal struggles.