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Duncan (23) won both races on the clay track at Agueda in Portugal over the weekend and, with the victories, went to the top of the leaderboard after two of the five rounds which make up the championships.
Riding her Kawasaki KX250F, she bagged the maximum 50 points from the two races and now sits four points clear of Dutch rider Nancy van de Ven at the top of the leaderboard.
In the first race of the weekend, Duncan exited the first turn in second place but, by the midway point of the first lap, she had taken the lead.
She was up by 2sec by the end of the opening lap.
She continued to stretch her lead and was more than 10sec ahead halfway through the race.
She eventually swept to victory by more than 12sec over German rider Larissa Papenmeier.
Duncan won the opening race of the world championships in the Netherlands last month, but then crashed heavily while leading race two and fought back to finish the race in seventh.
Unfortunately, she was forced to settle for fourth overall that weekend.
In race two yesterday, Duncan took the lead on lap two and quickly opened up a gap on her pursuers. In the end, Duncan finished more than 30sec ahead of runner-up van de Ven.
She was thrilled with the results.
"I got blocked in at the first turn in that second race, but I didn't panic, sorted out a good place to make a clean pass and could race away,'' she told media.
"I'm just taking it race by race at the moment and not really thinking about the championship.
"It was a great weekend - from qualifying on pole, then two race wins and taking the red [championship leader's] plate ... I really can't ask for much more.''
"Now it's a big break again [before the next round in the series in the Czech Republic], but I'll use this opportunity to continue to build. I'm very excited to get back to work and get better.''
"The amount of messages I've received from back home has been crazy, really. I appreciate it. I'll be going back to New Zealand to keep doing the work and come out swinging at Loket [in the Czech Republic].''
The next round of the championships is not until the end of July, so Duncan has plenty of time to rest and think about improvements. She is full of praise for her new British-based Kawasaki squad.
"They've given me an awesome opportunity and I'm very motivated to put them on top in 2019. A fresh start was what I needed and I couldn't be more excited than to do that with Kawasaki.
"Kawasaki New Zealand have also come on board to support me at home, for which I am also very thankful.''