Duminy hit 64 not out from just 39 deliveries to ensure that the Cobras chased down a target of 147 with eight balls to spare and finish six points ahead of the Dolphins, who lost their final game of the league stage against the Highveld Lions.
The Warriors were left with a sense of deja vu – having missed out on a One-Day Cup play-off by losing their final league game in that competition, they repeated the mistake on Sunday.
With Titans beating the Knights, the Warriors were again pipped at the post as the Titans set up a date with the Dolphins in Durban on Wednesday.
Although the Warriors managed to get after both Beuran Hendricks and Duminy after they had won the toss and elected to bat, they were unable to put a meaningful partnership together as wickets fell throughout the innings.
Hendricks conceded 40 runs from his four overs, but also picked up three wickets to extend his record for most scalps in a Twenty20 season – he now has 27 wickets at an average of 9.77.
Sunil Narine was economical again as he dismissed top scorer Davy Jacobs (33) and Wayne Parnell (27), but not as economical as Justin Kemp, who allowed just 11 runs off his four overs.
A score of 146 for eight looked below par on a good Newlands pitch, and once the Cobras had overcome an early hiccup in losing their openers for just 24 runs, Stiaan van Zyl (23) and Duminy constructed a 46-run stand which set the home side on their way.
Duminy was able to see the job through and was aided by Kemp, whose 26 not out included a couple of lusty blows.
The Cobras now have a week to prepare for the final, which will take place at Newlands next Sunday.