She's made a statement of intent in the opening round in Italy.
The 22-year-old from Palmerston was clearly the fastest rider in both races in Trentino on Monday (NZT), where she finished sixth and first.
A problem on the starting line left her more than 20 seconds behind the field in race one, before she gradually made up ground on her Yamaha, climbing from 25th to sixth in a race won by German Larissa Papenmeier.
There was no such problem in the second 12-lap race as Duncan led from start to finish, extending her winning margin to 22sec by the chequered flag.
She sits in a three-way share of second place behind Suzuki rider Papenmeier heading into the second of seven rounds in Portugal this weekend.
Duncan finished third overall last year, two points short of the title after a contentious jury decision in the final round in France saw her hopes scuppered. Italian Kiara Fontanesi claimed a fifth world title.
In 2016, Duncan led in her maiden campaign but missed two full rounds after suffering injury when she crashed into a track photographer at a round in Germany.
Katherine Prumm is the only Kiwi to have won the women's crown, in 2006 and 2007.
