An Etymological Dictionary of Astronomy and Astrophysics

1. Composed of two like parts or members.
   

2. Twice as large, heavy, strong, etc.
   

3. Anything that is twofold in size or amount or twice the usual size, quantity, strength, etc.

Etymology (EN): From O.Fr. duble, from L. duplus “twofold” (Gk. diplos “double”),
from duo “two” + -plus “fold.”

Etymology (PE): Dotâyi, from dotâ, from
do “two,” akin to L. duo, + tâ “fold, plait, ply; piece, part,” Mid.Pers. tâg “piece, part.”