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Duncan has a 21-point lead, having grabbed two second placings in the two races at Italy's Ottobiano circuit at the weekend.
The racing in Italy was in technical, sandy and hot conditions where temperatures reached up to 30degC.
Duncan (22) missed out on first place by less than one second in both her races.
The first race was a tough one. Duncan fell early but fought her way back to the front and pushed ahead of Italian leader Kiara Fontanesi. But the Italian was able to come straight back, retake the lead and win the race.
The first race was a mix of emotions for Duncan.
``I made a small mistake on the opening laps which made me really have to work for it. I made my way back to second right behind the leader. It was frustrating because I felt I let that win slip away, but we didn't give up and limited the points damage, so from that side I'm happy,'' she said.
The second race, on Sunday, was the first event on the track, so temperatures were lower.
Duncan kept applying the pressure on leader Nancy Van de Ven, of the Netherlands, but could not quite get past her and the Dutch woman managed to hold on for the win.
Fontanesi finished third.
Duncan said there were areas she would seek to improve on over the long break until round five in the Netherlands on September 16.
``I need to be better in the easier conditions and really work on my intensity in the first half of the race, as I feel like those are two of my weak points at the moment.
``However, 2-2 on a difficult weekend we can be happy with and actually, I extended my lead from 15 points to 21 in the standings, so from a championship perspective that's a real positive. Obviously, we can be better but I like the position I'm in and excited about the final two rounds.''
Duncan sits on 184 points and Van de Ven and Fontanesi are tied for second on 163.
Duncan will return to New Zealand for a few weeks early next month but will still have her eyes on the last two events of the world championships in the Netherlands and Italy.