Doppelgänger
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A Doppelgänger is a concept from German folklore referring to a ghostly double or look-alike of a living person, often used to describe an uncanny resemblance.
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Mentioned once in Mixed Doubles.
Mixed Doubles
Season 4 · Episode 6 · S04E06
“In steps a man who is Niles's Doppleganger — same height, same figure, same haircut, same neat suit, same rigid posture”