Duncan won both races in the latest round at Loket in the Czech Republic.
The 23-year-old Kawasaki rider had two excellent days and could not be caught over the whole weekend.
She was well ahead of other riders in the qualifying session - 2sec quicker than the next best rider - and then scored back-to-back wins on the hard-packed track, giving her an impressive tally of five race wins from six starts in the championships.
The flying Palmerston rider is now 14 points in front of Dutch rival Nancy Van De Ven with just two rounds remaining - at Imola, in Italy on August 17-18 and at Afyon, in Turkey on September 7-8.
Duncan crossed the finish line a massive 36sec ahead of Van De Ven in race one and then repeated the dose, also winning race two yesterday by 36sec, that time crossing the line ahead of German rider Larissa Papenmeier.
"It was a great weekend. Honestly, I couldn't really have asked for much more. I started on a good note by qualifying first, then went 1-1,'' she said.
"It was a tough race. The track's really technical and it was hot out there. I guess we could all feel it at the end of the race.
"I even fell off on the sighting lap, but it taught me to ride smart. Honestly, I felt super slow, but I won by 36 seconds, so it showed what a difficult race it was for everyone.
"I put another good stint in at home over the last month, so it's nice to see everything paying off.''
She said it was a good win for her British-based team and everyone in New Zealand.
"`It was pretty much a perfect weekend as I even set the fastest lap in practice, which is something I rarely do. I got two good starts and could immediately make good passes to lead every lap. This track is pretty tricky but I could stay concentrated and managed a good lead to the finish of each race.
"I've also got a good lead in the championship. Now so I just need to keep focused and bring it home.
"It was really nice to race with the 2020 bike this weekend. I tested it a couple of weeks ago and, after just 10 minutes, I knew it would be the bike I'd love racing with here.
"It's a new engine so the power is a lot different, but it's just so good and I could really feel the difference from my old bike.''
