A doppelgänger (; German: ['d?pl] , literally "double-goer") is a non-biologically related look-alike or double of a living person, sometimes portrayed as a ghostly or paranormal phenomenon and usually seen as a harbinger of bad luck. Other traditions and stories equate a doppelgänger with an evil twin. In modern times, the term twin stranger is occasionally used. The word "doppelgänger" is often used in a more general and neutral sense, and in slang, to describe any person who physically resembles another person.
The word doppelgänger is a loanword from the German Doppelgänger, a compound noun formed by combining the two nouns Doppel (double) and Gänger (walker or goer). The singular and plural forms are the same in German, but English usually prefers the plural "doppelgängers". The first known use, in the slightly different form Doppeltgänger, occurs in the novel Siebenkäs (1796) by Jean Paul, in which he explains his newly coined word by a footnote - while actually the word Doppelgänger also appears, but with a quite different meaning.
Like all nouns in German, the word is written with an initial capital letter. Doppelgänger and Doppelgaenger are essentially equivalent spellings, and Doppelganger is different and would correspond to a different pronunciation. In English, the word should be written with a lower-case letter (doppelgänger) unless it is the first word of a sentence or part of a title. It is further common to drop the umlaut on the letter "a", writing (and often pronouncing) "doppelganger".
English-speakers have only recently applied this German word to a paranormal concept. Francis Grose's, Provincial Glossary of 1787 used the term fetch instead, defined as the "apparition of a person living." Catherine Crowe's book on paranormal phenomena, The Night-Side of Nature (1848) helped make the German word well-known. However, the concept of alter egos and double spirits has appeared in the folklore, myths, religious concepts, and traditions of many cultures throughout human history.
In Ancient Egyptian mythology, a ka was a tangible "spirit double" having the same memories and feelings as the person to whom the counterpart belongs. The Greek Princess presents an Egyptian view of the Trojan War in which a ka of Helen misleads Paris, helping to stop the war.. This is depicted in Euripides' play Helen. In Norse mythology, a vardøger is a ghostly double who is seen performing the person's actions in advance. In Finnish mythology, this is called having an etiäinen, "a firstcomer". The doppelgänger is a version of the Ankou, a personification of death, in Breton, Cornish, and Norman folklore.
With the advent of social media, there have been several reported cases of people finding their "twin stranger" online, a modern term for a doppelgänger. Twinstrangers.net is a website where users can upload a photo of themselves and facial recognition software attempts to match them with another user of like appearance. The site reports that it has found numerous living doppelgängers--including three living doppelgängers of its founder Niamh Geaney.
In addition to describing the doppelgänger double as a counterpart to the self, Percy Bysshe Shelley's drama Prometheus Unbound makes reference to Zoroaster meeting "his own image walking in the garden".
In The Devil's Elixir (1815), a man murders the brother and stepmother of his beloved princess, finds his doppelgänger has been sentenced to death for these crimes in his stead, and liberates him, only to have the doppelgänger murder the object of his affection. This was one of E. T. A. Hoffmann's early novels.
Fyodor Dostoyevsky's novel The Double (1846) presents the doppelgänger as an opposite personality who exploits the character failings of the protagonist to take over his life. Charles Williams' Descent into Hell (1939) has character Pauline Anstruther seeing her own doppelgänger all through her life.Clive Barker's story "Human Remains" in his Books of Blood is a doppelgänger tale, and the doppelgänger motif is a staple of Gothic fiction.
In Stephen King's book The Outsider, the antagonist is able to use the DNA of individuals to become their near perfect match through a science-fictional ability to transform physically. The allusion to it being a doppelganger is made by the group trying to stop it from killing again. The group also discusses other examples of fictional doppelgangers that supposedly occurred throughout history to provide some context.
In Bret Easton Ellis's novel, Glamorama, protagonist actor-model Victor Ward, ostensibly, has a doppelgänger that people mistake for Ward, often claiming to have seen him at parties and events Ward has no recollection of attending. At one point in the novel, Victor heads to Europe but reports of him attending events in the states appear in newspaper headlines. However, Victor's doppelgänger may or may not have been placed by Victor's father, a United States senator looking to present a more intelligent and sophisticated replacement for his son that would improve his own image and boost his poll numbers for future elections. While the novel is narrated by Victor, various chapters are ambiguous, leading the reader to wonder if certain chapters are being narrated by the doppelgänger instead.
In Richard Ayoade's The Double, based on Fyodor Dostoevsky's novel of the same name, a man is troubled by a Doppelgänger who is employed at his place of work and affects his personal and professional life.
The 1969 film Doppelganger involves a journey to the far side of the sun, where the astronaut finds a counter-earth, a mirror image of home. He surmises his counterpart is at that moment on his earth in the same predicament.
In The CW supernatural drama series, The Vampire Diaries, actress Nina Dobrev portrayed the roles of several doppelgangers; Amara Petrova (the first doppelganger), Tatia Petrova (the second), Katerina Petrova/Katherine Pierce (the third) and Elena Gilbert (the fourth). The series mainly focused on the doppelgangers of the sweet & genuine Elena and the malevolent Katherine. In the same series, Paul Wesley portrays Silas and his doppelgangers Tom Avery and Stefan Salvatore.
In The Simpsons episode "Fear of Flying", Homer Simpson is banned from entering Moe's Tavern. A man enters the bar afterwards looking like Homer with a high hat and a moustache, claiming to be Guy Incognito. As he is beaten up and thrown out by the patrons, who were convinced that the man was a disguised Homer, the real Homer passes by and notices, rather casually, that he found his doppelganger, only to be distracted by a dog with a puffy tail. 
In the sitcom How I Met Your Mother, throughout the fifth and sixth seasons, the five main characters each encounter an identical stranger of themself. By the episode "Double Date", they have spotted Marshall's doppelganger, nicknamed "Moustache Marshall", and Robin's, called "Lesbian Robin". In the same episode they find Lily's doppelganger, a Russian stripper named Jasmine. Later, in the episode "Robots Versus Wrestlers", the gang finds Ted's double, a Mexican wrestler, but Ted himself is not there to witness it. In "Doppelgangers", Lily and Marshall decide that as soon as they find Barney's doppelganger, it will be a sign from the universe for them to start trying to have children. Lily spots a pretzel vendor who she thinks looks like Barney, but in reality looks nothing like him. Marshall takes this mistake as Lily subconsciously affirming her desire for motherhood and they decide to start trying for a baby. They meet Barney's real doppelganger, Dr. John Stangel, in the episode "Bad News", though they initially think he is simply Barney in disguise.
Heautoscopy is a term used in psychiatry and neurology for the hallucination of "seeing one's own body at a distance". It can occur as a symptom in schizophrenia and epilepsy, and is considered a possible explanation for doppelgänger phenomena.
Criminologists find a practical application in the concepts of facial familiarity and similarity due to the instances of wrongful convictions based on eyewitness testimony. In one case, a person spent 17 years behind bars persistently denying any involvement with the crime of which he was accused. He was finally released after someone was found who shared a striking resemblance and the same first name.
In one of the stranger twists of fate in literary history, Jean Paul coins two terms in Siebenkäs, "Doppelgänger" and "Doppeltgänger." The term Jean Paul uses to describe Siebenkäs and Leibgeber is "Doppeltgänger," which he defines in a footnote: "So heißen Leute, die sich selber sehen" ["the name for people who see themselves"] (2, 67). Earlier in Siebenkäs the neologism "Doppelgänger" also appears for the first time and means something quite different. In a description of the wedding banquet in the first chapter, the food is so delicious and abundant that "not only was one course [Gang] served but also a second, a Doppelgänger" ["nicht bloß ein Gang aufgetragen wurde, sondern ein zweiter, ein Doppelgänger"] (2, 42). "Gang" in German has multiple meanings, ranging from a "walk" to the "course" of a meal; according to Jean Paul, when people "see themselves," when one "goes twice," one is a "Doppeltgänger"; when one has a meal of two courses, in which the second doesn't come second but together with the first, this is a "Doppelgänger."