doppelgänger is defined by Oxford Learner's Dictionaries, which labels the noun "(from German)," this way: "a person’s doppelgänger is another person who looks exactly like them."
Cambridge Dictionary, marking the noun countable, defines it as "a spirit that looks exactly like a living person, or someone who looks exactly like someone else but who is not related to that person."
Dictionary.com's American entry, which also gives the capitalized form Doppelgänger, defines it as "a ghostly double or counterpart of a living person." Its British entry, from Collins English Dictionary, labels the sense "legend" and gives "a ghostly duplicate of a living person." Dictionary.com's etymology reads "First recorded in 1850–55; from German: literally, “double-walker”."