1. to do two jobs at one time.

Kudrow does double duty in the show, playing her regular role and the character's twin sister.

2. to be used for two different activities.

Three conference rooms do double duty as dining rooms when the main dining room is full.

• be on someone's back
• be on someone's case
• be on someone's shit list
• be on song
• be on speaking terms
• be on tail
• be on the back burner
• be on the ball
• be on the blink
• be on the bottle
• be on the breadline
• be on the brink of
• be on the brink of doing something
• be on the cards
• be on the case
• be on the crest of a wave
• be on the [expensive/heavy/large etc.] side
• be on the expensive side
• be on the fiddle
• be on the fritz
• be on the game
• be on the heavy side
• be on the horns of a dilemma
• be on the large side
• be on the level