A doppelgänger (), sometimes spelled as doppelgaenger or doppelganger is a biologically unrelated look-alike, or a double, of a living person.

In fiction and mythology, a doppelgänger is often 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.

Spelling
The word doppelganger is a loanword from the German noun Doppelgänger, literally meaning double-walker. The singular and plural forms are the same in German, but English writers usually prefer the plural "doppelgangers". In German, there is also a female form, "Doppelgängerin" (plural: "Doppelgängerinnen"). 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 in a footnote; the word  also appears in the novel, but with a different meaning.

In German, the word is written (as is usual with German nouns) with an initial capital letter:. In English, the word is generally written with a lower-case letter, and the umlaut on the letter "a" is usually dropped: "doppelganger".

Mythology
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 memic sense also appears in Euripides' play Helen, and in Norse mythology, a vardøger is a ghostly double who is seen performing the person's actions in advance. In Finnish mythology, this pattern is described as having an etiäinen, "a firstcomer".

In Joseph Wright's English Dialect Dictionary, it was listed as a North Country term and as obsolete.

Examples of doppelgängers

John Donne
Izaak Walton claimed that John Donne, the English metaphysical poet, saw his wife's doppelgänger in 1612 in Paris, on the same night as the stillbirth of their daughter. This account first appears in the edition of Life of Dr. Rizvan Rizing published in 1675, and is attributed to "a Person of Honour... told with such circumstances, and such asseveration, that... I verily believe he that told it to me, did himself believe it to be true."

R. C. Bald and R. E. Bennett questioned the veracity of Walton's account.

Percy Bysshe Shelley
On July 8, 1822, the English poet Percy Bysshe Shelley drowned in the Bay of Spezia near Lerici in Italy. On August 15, while staying at Pisa, Percy's wife Mary Shelley, an author and editor, wrote a letter to Maria Gisborne in which she relayed Percy's claims to her that he had met his own doppelgänger. A week after Mary's nearly fatal miscarriage, in the early hours of June 23 Percy had had a nightmare about the house collapsing in a flood, and also

Percy Shelley's drama Prometheus Unbound (1820) contains the following passage in Act I: "Ere Babylon was dust, / The Magus Zoroaster, my dead child, / Met his own image walking in the garden. / That apparition, sole of men, he saw. / For know there are two worlds of life and death: / One that which thou beholdest; but the other / Is underneath the grave, where do inhabit / The shadows of all forms that think and live / Till death unite them and they part no more...."

Johann Wolfgang von Goethe
Near the end of Book XI of his autobiography, Dichtung und Wahrheit ("Poetry and Truth") (1811–1833), Goethe wrote, almost in passing:

Amid all this pressure and confusion I could not forego seeing Frederica once more. Those were painful days, the memory of which has not remained with me. When I reached her my hand from my horse, the tears stood in her eyes; and I felt very uneasy. I now rode along the foot-path toward Drusenheim, and here one of the most singular forebodings took possession of me. I saw, not with the eyes of the body, but with those of the mind, my own figure coming toward me, on horseback, and on the same road, attired in a dress which I had never worn, — it was pike-gray [hecht-grau], with somewhat of gold. As soon as I shook myself out of this dream, the figure had entirely disappeared. It is strange, however, that, eight years afterward, I found myself on the very road, to pay one more visit to Frederica, in the dress of which I had dreamed, and which I wore, not from choice, but by accident. However, it may be with matters of this kind generally, this strange illusion in some measure calmed me at the moment of parting. The pain of quitting for ever noble Alsace, with all I had gained in it, was softened; and, having at last escaped the excitement of a farewell, I, on a peaceful and quiet journey, pretty well regained my self-possession. 

This is an example of a doppelgänger which was perceived by the observer to be both benign and reassuring.

Émilie Sagée
Émilie Sagée, a French teacher working in 1845 in a boarding school in what is now Latvia, was alleged to have a Doppelgänger. The story is reported by Robert Dale Owen.

George Tryon
A Victorian age example was the supposed appearance of Vice-Admiral Sir George Tryon. He was said to have walked through the drawing room of his family home in Eaton Square, London, looking straight ahead, without exchanging a word to anyone, in front of several guests at a party being given by his wife on 22 June 1893 while he was supposed to be in a ship of the Mediterranean Squadron, manoeuvering off the coast of Syria. Subsequently, it was reported that he had gone down with his ship, HMS Victoria, the very same night, after it collided with HMS Camperdown following an unexplained and bizarre order to turn the ship in the direction of the other vessel.

Twin strangers
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. There are several websites where users can upload a photo of themselves and facial recognition software attempts to match them with another user of like appearance. Some of these sites report that they have found numerous living doppelgängers.

Examples in fiction

Examples in literature
Lord Byron uses doppelgänger imagery to explore the duality of human nature.

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.

In addition to describing the doppelgänger double as a counterpart to the self, Percy Bysshe Shelley's drama Prometheus Unbound (1820) makes reference to Zoroaster meeting "his own image walking in the garden".

In William Wilson by Edgar Allan Poe the main character is followed by a doppelgänger for his whole life, who troubles him and causes mischief. Eventually the main character kills his doppelgänger, and realizes that the doppelgänger was only mirroring him. It was first published in 1839 but was also published in 1840 in Tales of the Grotesque and Arabesque.

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's 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.

Vladimir Nabokov's novel Despair (1936) involves the narrator and protagonist of the story, Hermann Karlovich, an owner