Green’s Dictionary of Slang

(US) lit. or fig., to obtain a double profit for a single investment.

1990		T. Pluto Loose Balls 179: I know of several cases where an agent double-dipped. By that, I mean the agent took his cut from the kid after doing the contract, then he took money on the side from the ABA to deliver the kid.
2019		S.M. Jones Lives Laid Away [ebook] ‘The rogue ICE units look like they’re only responsible for moving women out. Maybe double-dipping selling some of the women as drug mules’.