An Etymological Dictionary of the German Language/Annotated/doppelt

by Friedrich Kluge

doppelt, adjective (a parallel form, Doppel, occurs in the compounds Doppeladler, Doppelgänger), ‘double, duplicate, twofold,’ Modern High German only, from French double; Middle High German dublin, ‘double,’ is a derivative from the same source. The final t of the Modern High German word is a secondary suffix, as in Art, Obst.