The Senegalese star gave Watford a 12th-minute lead with a left-footed drive past Reading goalkeeper Rafael Cabral.
He made it 2-0 within 77 seconds with a blistering shot from a tight angle.
Watford, relegated from the top flight last season, stays second, five points behind leader Norwich City.
Crucially, the Hornets have an imposing 12-point lead over third-place Brentford.
Reading remains in the final play-off slot in sixth place but is in danger of being overtaken by Bournemouth, which now has two games in hand.