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The 22-year-old took the lead in the world standings after she comprehensively won both races in contrasting conditions at the Portuguese leg staged in Agueda.
``It was really tricky conditions out there and both races were completely different,'' Duncan said.
``One was heaps of ruts and really technical and the second moto was really muddy, as we got rain in the night, which made for tough conditions out there this morning.''
Duncan came into the weekend's racing tied for second placing in the championship standings after a fast but trouble-plagued start to the competition.
Trouble on the starting line in the first of two races in Italy got her 2018 season off to a terrible start, but Duncan fought back to win the second race in Pietramurata.
She carried on her winning way and made it a hat-trick of wins in Portugal and notched her first ever winning double at a World Championship leg.
Duncan posted the fastest lap during practice and then carried that momentum into the weekend's first race.
``It was a really good weekend and I'm stoked with how it ended up. I was on pole in qualifying, then was able to go 1-1 in both races,'' Duncan said.
``To come away with two wins and the championship lead is super rewarding.''
After leading early in race one, Duncan beat the Netherlands' Nancy Van De Ven by 13.9sec. She was followed home by Germany's Larissa Papenmeier.
In race two, Papenmeier shot into the lead ahead of reigning world champion Kiara Fontanesi, of Italy. After the first lap, Duncan made it past Fontanesi for second and had her eyes set on Papenmeier. Duncan took the lead on the fifth of 10 laps and went on to beat Papenmeier by 8.7sec.
Duncan has returned to her base in Belgium to prepare for the next leg of the World Motocross Championship in Germany on May 20.