Fifth-placed Duffield were unable to halt leaders Dunstall after a 107-run defeat the high-flyers.
Chasing 245 for six, the visitors were bowled out for 138 in the 46th over.
The run-chase got off to the worst possible start when Duffield slumped to 15 for three, losing both openers cheaply.
They recovered through a 52-run fourth-wicket partnership between Peter Jenkins and Jack Clark.
Both batsmen made 27 and struck four boundaries.
Clark was dismissed at 67 and Jenkins three runs later.
David Whiteley (13) and captain Thomas Roe launched another fightback.
They added 22 for the sixth wicket before two wickets fell in quick succession.
Roe, the wicketkeeper, continued to battle and struck seven boundaries in his 32 before he was eighth man out on 123.
The visitors were eventually dismissed for 138.
Earlier Duffield’s bowlers made a terrific start, reducing the leaders to 32 for four through Joe Wozniczka-Wells (two for 51, from 15 overs) and James Hardwick (three for 60, from 13 overs).
Dunstall recovered, however, through a 137-run stand for the fifth wicket as opener James Benstead carried his bat for 106 not out.