1) Barreledbarreled 2) Binal 3) Diploid 4) Double 5) Doubled 6) Doubly 7) Dualistic 8) Duple 9) Multiple 10) Siamese 11) Tweyfold 12) Twice 13) Twifold 14) Twin 1) By a factor of two 2) Double 3) Doubly 4) Dual 5) Multiple 6) Threefold 7) Treble 8) Twice as great or many 
• (a.) Double; duplicate; multiplied by two; as, a twofold nature; a twofold sense; a twofold argument. • (adv.) In a double degree; doubly.
[adv] - by a factor of two
adverb by a factor of two; `the price increased twofold last year`