The Proteas all-rounder first struck a busy 68 off 62 balls as the hosts reached 265/7 after winning the toss and batting first. Duminy was then the pick of the bowlers with 2/19 as the visitors were bowled out for 197 in 44.4 overs.
Veteran opener Andrew Puttick, fresh from paternity leave, top-scored for the Cobras with a flowing 79 off 89 balls, while debutant all-rounder Jason Smith looked at ease through his run-a-ball 50. Ayavuya Myoli was the pick of the Dolphins bowlers with 3/40, while Imran Tahir returned figures of 2/51 ahead of his return to national duty. The Durban side were in trouble even before a ball could be bowled as Imraan Khan could not bat, meaning Tshepang Dithole joined the fit-again Morne Van Wyk at the start.
But both fell inside eight overs as the visitors struggled to find any rhythm. Their best hope, David Miller, was run out for 42, and despite some lusty late lower-order blows from top-scorer Ryan McLaren (52 off 55), the Dolphins fell woefully short. There was also a noteworthy return from injury for Proteas left-arm seamer Beuran Hendricks as he claimed 2/46.
In the end it was a bonus-point win for the Cobras, who moved to 19 points at the summit, with the Dolphins dropping to second-bottom on nine points.