Heading into today's final grand prix in Trentino, Italy's far north, the Dunedin rider held a 16-point lead over local Fontanesi - a six-time world champion.
Fontanesi's hopes of chasing down the Kiwi however ended on the first hill of race one when she was taken out by Britt Jans-Bekenr of Netherlands. Fontanesi got back on her bike and recovered to finish 15th but Duncan secured the title by finishing third behind German Larissa Papenmeier and Van De Ven.
Duncan found her flow in the second half of the race to work her way onto the podium - enough to claim the title with a race to spare.
The celebrations began early but they didn't seem to stop her momentum in race two. Fontanesi capped off a disappointing home grand prix when she clipped Duncan's wheel to fall down the field from third, while the recently crowned world champion cruised by Papenmeier to eventually win by eight seconds.