Doppelgangers were evil counterparts of individuals, originating from the Black Lodge.
During his time in the Black Lodge, Dale Cooper encountered doppelgangers of Laura Palmer, Leland Palmer, the arm, Annie Blackburn, Maddy Ferguson, Caroline Earle, and himself. His own doppelganger escaped from the Lodge and assumed his identity.
Doppelgangers had identical appearances to their counterparts, with the exception of their eyes. Within the Lodge, their irises were white, and outside the Lodge, they were black.
Behind the scenes
The only times the word is used in the franchise, it is written "doppelganger" without umlauts, but in other fiction and folklore it is written "doppelgänger". It might be because the font used does not support such characters but since this font is not yet identified, it remains unsure.
A doppelgänger is a paranormal double of a living person, typically representing evil or misfortune. In modern vernacular, it is simply any double or look-alike of a person, as is the original German meaning of the word.