Definitions
1.verbTo double the quantity, amount or duration of something"I'm going to double up my enlistment."
2.verb(by extension) To double one's amount of chips by winning an all-in pot.
3.verbTo bend, bend over; to fold; to stoop.
4.verbTo have a secondary use."This unfolding sofa doubles up as a bed."
5.verbTo get the second out in a double play, typically referring to getting an out by beating a runner back to a base (often by throwing) after a fly ball has been caught
6.verbTo employ double the usual resources for a particular purpose.