She won the first race and then finished second yesterday in the second race to win the Netherlands WMXGP at Assen.
Duncan won the first race easily on Sunday and, in the second race, led for most of the way.
She lapped most of the field in an impressive display.
However, she got stuck in the ruts of the course and went off the track.
``I'm so gutted. I had a lead of like 30sec, but I crashed and then went off the track and because it's been built up, it was hard to get back on,'' Duncan told local media.
Dutch rider Nancy Van de Ven took the lead with two laps to go and managed to keep ahead of Duncan.
Duncan set the fastest times in both races but, such was the roughness of the track on the second day, the fastest time was more than 30sec slower than on the first day.
Van de Ven won by just over 5sec from Duncan with world champion Livia Lancelot, of France, in third.
Duncan went up to fifth in the overall standings, on 196 points.
Lancelot's consistency in finishing on the podium in all seven rounds was the key to her championship success.
Duncan (20) did have the consolation of her manufacturer, Yamaha, winning the WMX manufacturers' title.
Duncan will now return home to New Zealand to get her body right and ride in some events over the coming summer before switching focus for next year's world championship.
After a great start to the season, she received a nasty hand injury when she collided with a photographer in Germany in May which scuttled her hopes of winning the title.
