Definitions:
consisting of two equal, identical, or similar parts or things

lower in pitch by an octave.

twice as much or as many

at or to twice the amount or extent

a thing which is twice as large as usual or is made up of two standard units or things

a person who looks exactly like another

(especially in tennis and badminton) a game or competition involving sides made up of two players

a system of change-ringing using five bells, with two pairs changing places each time.

a pair of victories in the same sport in two different competitions

a number or amount which is twice as large as a contrasting or usual number or amount

become twice as much or as many

fold or bend (paper, cloth, or other material) over on itself

be used in or play another, different role