The Yamaha team rider won against both men and women at the 55th running of the New Zealand Motocross Grand Prix at Woodville over the weekend.
Just back from knee surgery which had put her out for nearly a year, Duncan had two class wins at Woodville, which is New Zealand's biggest stand-alone motocross event.
Duncan, who turned 20 last week, lapped the entire field in each of her three women's class races, winning the day ahead of fellow Yamaha rider Jessie Waterhouse, from Tauranga.
With little time to refuel her bike and replace the damaged air filter, she was back on the start line to tackle the MX2 men.
She had to race against New Zealand internationals Josiah Natzke, of Hamilton, and Hamish Harwood, of Takaka.
Natzke was European 125cc No2 and New Zealand 125cc champion last season, while former Australian under-19 champion Harwood was national MX2 No2 last year, but they were both humbled by the young woman from Palmerston.
Duncan produced a 1-3-2 score-line in the three races in the MX2 class, edging out Natzke by four points. Harwood was another three points further back.
Duncan was thrilled with the performance but was not getting too carried away.
"The day went really well,'' she said.
"I had three reasonably good starts (in the MX2 class) and then just set about riding my own race. Everyone is fast in this class, but I just wanted to focus on my own riding.
"My fitness has definitely improved over the past few weeks and working with [Altherm JCR Yamaha Racing Team boss] Josh Coppins has given me a great boost.''
She said it was great to perform in race conditions after not being exposed to racing for so long.
She had no idea how she had done at the end of each race and did not know she had lapped the whole of the women's field.
Training against her team-mate, Australian Dean Ferris, was a real bonus.
"It just brings a lot more intensity to everything. Just racing all the time, working hard day in, day out on the bike.''
Duncan will continue to train, but her big aim is the first round of the women's world motocross championships in Qatar on February 27-28.
"I don't really know what to expect when I get there. I don't know any of the girls and I've never come up against them.''
If she performs well in this event she is likely to line up in the rest of the championships, which are in Europe over the northern hemisphere summer.
