To double the quantity, amount or duration of something
I'm going to double up my enlistment.
(by extension) To double one's amount of chips by winning an all-in pot.
To bend, bend over; to fold; to stoop.
To have a secondary use.
This unfolding sofa doubles up as a bed.
To 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
To employ double the usual resources for a particular purpose.