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The 22-year-old has won both races of the championship's latest round in Germany.
The Otago rider beat defending champion Kiara Fontanesi - who was second in both races - by seven and 15 seconds.
Fontanesi shot out to an early lead in the first race, leaving Duncan trailing her for 10 laps.
With two laps to go, Duncan made a pass and raced ahead to take a win by 7sec.
``I wasn't really willing to push it on the first few laps,'' Duncan said.
``The track was difficult in its own ways. It was very hard to pass and rock-hard.
``It took me a while to find my flow.''
Indeed she had been considering protecting her points and accepting second place at one point.
``Then I was like `nah, we are going to go for it'. I still felt good enough to win.''
The second race was more comprehensive.
Duncan took the early lead, with Fontanesi trailing in third and Nancy van de Ven splitting the duo.
Fontanesi worked her way back into second, although Duncan's lead was far too much to overcome.
``I took the holeshot and just rode smart to bring it home and was able to pull a 15sec lead on Kiara in the end, so that was cool.
``I'm really happy to come out of here having extended out my points lead in the championship.''
Duncan holds a 15-point lead in the overall championship, which is now halfway through.
In second place is Larissa Papenmeier, while Fontanesi and Van de Ven are tied in third, 22 points behind.
Duncan is set to compete in France in a fortnight, before the next round of the world championships in Italy on June 16 and 17.