South Africa [ Images ], who clinched the series 2-0, piled up 194 for six and Zimbabwe made a spirited reply with a national record total of 186 for seven.
Duminy's first 50 came from a measured 37 balls but his next 17 deliveries produced 46 runs, the middle-order batsman hitting 10 fours and four sixes.
Opener Graeme Smith [ Images ] contributed 46 from 29 balls to add to his 58 in the first match in Bloemfontein which South Africa won by seven wickets.
Zimbabwe refused to be overawed by the daunting total and were ahead of the asking rate at the halfway point thanks to half centuries from opener Brendan Taylor (59 from 39 balls) and Chamu Chibhabha (58 from 41).
But crafty death bowling from Rusty Theron stifled the chase and earned the seamer with a reputation for producing yorkers on demand figures of two for 26 from his four overs.
The teams begin a three-match one-day international series in Bloemfontein on Friday.