a double (binary) star.
a double (or dual) role for an actor.
an egg with a double yolk.
ancient Greek had the dual form but it has merged with the plural form in modern Greek.
double doors.
dual controls for pilot and copilot.
duple (or double) time consists of two (or a multiple of two) beats to a measure.
every episode has its double and treble meaning.
the office of a clergyman is twofold.
public preaching and private influence.