The Palmerston-based rider stunned the crowd when she won the class's first race on Sunday riding her Yamaha YZF250, with nothing between her and Altherm JCR Yamaha team-mate Kayne Lamont, of Mangakino.
Results were reversed in the next race, with 19-year-old Duncan leading until Lamont gained the edge on a last lap pass and held off a spirited response from her to level the scores.
The third and final race of the day would be the MX2 class decider but with Duncan also racing (and comfortably winning) the women's class, it meant her workload was much greater and her energy was rapidly being sapped.
The heat and the roughness of the track were also contributing factors, so when Lamont holeshot the last MX2 race, it was almost impossible for her to claw him back.
Lamont said he and Duncan were "feeding off each other a lot today, talking with one another about what lines were best and how to cut our lap times''.
"We both wanted to win but we are also team-mates and helping each other out,'' he said.
For Duncan, it was her first race in 10 months, after recovering from a knee injury, and as always she enjoyed the heightened challenge of battling it out with the blokes.
"All in all it was good to be out there and the results were just a bonus,'' she said.
Queenstown's Scott Columb was runner-up in the feature race behind Mount Maunganui's Cody Cooper.
Duncan will travel to Qatar next month on a mission to win the Women's World Championships.
In the meantime, she races again in the women's and MX2 categories at the New Zealand Motocross Grand Prix at Woodville this weekend.
