Duckett had already scored a hundred against Pakistan A earlier in the tri-series tournament, while Daniel Bell-Drummond became a fourth centurion for the Lions with an unbeaten 171 off 139 balls as he and Duckett shared a staggering 367-run stand.
Sri Lanka’s only success came as Ramith Rambukwella (1-83) picked up Lions skipper Dawid Malan (23), stumped, but the rest of the attack went wicketless, including seamer Thisara Perera, whose 10 overs cost 101 runs.
With the bat, the visitors showed a lot more fight, with Niroshan Dickwella (60) and Mahela Udwatte (40) sharing in a century opening stand.
Sussex left-armer George Garton removed both in the same over – on his way to impressive figures of 4-43 – as Sri Lanka suddenly lost five wickets for 63 runs.
Perera went some way to making up for his disastrous day with the ball by blasting 45 from 40 balls before he too fell to Garton, bowled, and Sri Lanka ultimately were bowled out for 285 in 47.3 overs. (Courtesy Sky Sports)