Neal Maupay latched onto Yves Bissouma’s interception for his second goal in as many games, as Brighton secured a perfect league start of two wins from two.
Frenchman Maupay was forced out of action at half-time with a suspected shoulder injury, however, that was the sole blight on a hugely impressive shift from Graham Potter’s men.
Duffy’s first Premier League goal since November 2019 put Brighton in the driving seat, with the Ireland defender a popular scorer after a testing 18 months.
The 29-year-old spent last term on loan at Celtic after making just 12 starts in two years under boss Potter at Brighton.
But the death of his father and other off-field issues led to an error-strewn stint in Scotland, and Duffy did not start for the Hoops after February.
Duffy returned to Brighton in the close season expecting to head out on loan again, only for his Seagulls status to turn entirely on its head.
Duffy might have impressed in last weekend’s 2-1 win at Burnley, but his towering header here cemented his hard-earned return to form.
Alongside Webster and skipper Dunk in defence, Duffy helped Potter’s men shut out a Watford side who could not cope with Potter’s tactical blueprint.
Brighton started at breakneck speed and could have plundered the lead straight from kick-off.
A promising raid petered out, but the Hornets failed to heed the immediate warning signs and the Seagulls refused to relent on that punishing pace.
Pascal Gross’s whipped corner begged for a powerful connection, and Duffy delivered in style.
The revitalised centre-back thundered his header home off the underside of the bar, leaving Watford stunned by the hosts’ early ferocity.
Potter’s fluid set-up posed constant first-half problems for a stressed and stretched Watford, with Leandro Trossard, Alexis Mac Allister and Gross all threading passes to unpick the visitors.
Brighton finally made that dominance pay thanks to Bissouma’s relentless graft. The Mali midfielder punished Watford’s sloppy attempt to play out from the back, with a sweeping interception that quickly turned into an assist.
Maupay raced onto Bissouma’s wily intervention, steadied himself and wrong-footed Watford keeper Daniel Bachmann to double the home side’s lead.
The French striker doubled his tally for the season in the process, having netted in last weekend’s win at Burnley.
Aaron Connolly replaced Maupay at the interval and promptly spurned a gilt-edged chance, skewing high and wide with the goal at his mercy from eight yards out.
Emmanuel Dennis thought he had dragged Watford back into the tie after bustling through and slotting past Robert Sanchez, only for the realisation to hit that he had long since been flagged offside.
But the bright sparks were few and far between for a Watford side seemingly caught out by Brighton’s hybrid formation.
The Seagulls’ mix of 3-4-4 and 4-3-3 will quickly become familiar to the Hornets given its current vogue, and manager Xisco Munoz will have to devise a smarter counter plan than that on offer on the south coast.