There was a frantic start, with Peter Etebo’s loose pass moments after kick-off encouraging the hosts to attack right from the off, and it was a pace that persisted during the opening stages.
There was some brief respite when Ken Sema drove into the area and forced a Watford corner, but the set-piece came to nothing and Brighton were about to take a 10th-minute lead from a corner of their own.
Duffy outjumped Adam Masina to fire a bullet header onto the underside of the bar, leaving Daniel Bachmann no chance as it bounced over the line in an early blow for the Hornets.
Ismaïla Sarr fancied his chances of a quick equaliser, collecting a loose ball 20 yards from goal and shooting high of the target, but it was a rare sight of goal for an otherwise second-best Watford side during the opening half.
Despite their dominance, there was an element of luck to Brighton’s second goal. William Troost-Ekong under-hit a pass to Tom Cleverley, allowing Yves Bissouma to nip in and tee up Maupay, who had the simple task of slotting past Bachmann four minutes shy of the break.
Watford needed something to change, and Xisco Muñoz opted to withdraw Imrân Louza 45 minutes in to his first Premier League game. Hernández took his place, with Ken Sema dropping into a midfield three deprived of the injured Juraj Kucka as well as Dan Gosling and Nathaniel Chalobah, both suffering with sickness.