Green’s Dictionary of Slang

(US drugs) of a crack cocaine dealer, to offer ‘two for the price of one’.

1985		T.R. Houser Central Sl. 69: double-up halves A method of wholesaling rock cocaine.
1995		Bone Thugs-N-Harmony ‘1st Of Tha Month’ 🎵 I get wit my nigga to get me some llello / Double up nigga what you need? / We got weed to get P.O.Ded / Fiend for the green leaves.
2001		ONDCP Street Terms 8: Double up [...] Dub — When a crack dealer delivers an extra rock as a marketing ploy to attract customers.