An Etymological Dictionary of the German Language/doppelt

by Friedrich Kluge

doppelt, adj. (a parallel form, Doppel, occurs in the compounds Doppeladler, Doppelgänger), ‘double, duplicate, twofold,’ ModHG. only, from Fr. double; MidHG. dublin, ‘double,’ is a deriv. from the same source. The final t of the ModHG. word is a secondary suffix, as in Art, Obst.