The Otago seamer took five for 18 to help set up a nine-wicket win against Canterbury in a twenty20 fixture in Christchurch yesterday.
While it was the 21-year-old's maiden five-wicket haul in the format, he had a breakthrough season in the Plunket Shield last year with a tournament-high 45 wickets, including three five-wicket bags.
He was at his best again yesterday, getting the ball to swing plenty, and he ripped through to middle order to help dismiss Canterbury for 134.
He bowled a rare wicket maiden - there is not even a column for that now - and also picked up the key wicket of Henry Nicholls, who was threatening to bat his side back into the match.
Fellow seamer Warren Barnes (two for 34) and off spinner Nathan McCullum (one for 24) played their part to help reduce Canterbury to 28 for five.
Barnes continues to impress since moving south from Auckland this season and captured one of the bigger scalps when he removed Peter Fulton for one. He also bowled the dangerous Ronnie Hira for a duck.
The Kings rallied through Nicholls (51 from 41) and Todd Astle (28 from 33).
The pair put on 73 for the sixth wicket, which was a Canterbury record against Otago, eclipsing the previous mark of a paltry 23.
Duffy made the catch that broke the stand and then returned to the bowling crease to complete his remarkable night with three wickets at the death.
''He bowled very nicely,'' Otago coach Nathan King said.
''He had the ball swinging again and hit really good areas. It was a great spell from him.
''They might look at it and say they batted poorly, but in my eyes it was quality bowling.''
The experienced Neil Broom paced the chase beautifully with an undefeated 42 from 41, while Michael Bracewell provided the punch with a hard-hit and undefeated 80 from 49 deliveries.
It was Bracewell's highest score in the format - his previous best was 61 - and his partnership with Broom of 133 set an Otago-record second-wicket partnership against Canterbury.
''It was a complete performance really - a re-run of our first effort,'' King said.
''It was a great bowling effort, we fielded very well and, obviously, we got the job done with the bat.''
Otago returns home to Dunedin and will host Wellington at the University Oval on Sunday.
