She crashed out in the first race but managed to shrug that off to come back and win the second race in Ernee in the northwest of the country.
She has relinquished the lead in the championship but is still handily placed.
The 21-year-old led out in the first race, before her luck took a turn for the worse. She hit the dirt initially but managed to get back and up to second place.
However, while trying to get to the front she crashed badly and was out of the race.
That race was on Saturday and, after having her shoulder checked at hospital, she was back on the starting line for the second race on Sunday.
She certainly came back with a fire in her belly, making her way to the front by the second lap even though she started in 10th.
Frenchwoman Livia Lancelot had taken the best position at the start but she was quickly passed by Nancy Van De Ven, of the Netherlands, on the first jump.
Lancelot did not let Van De Ven lead for long though and, by the end of the first lap, was back in first.
After Duncan's pass on Lancelot, it was a game of cat and mouse for the lead but the Frenchwoman knew she had the overall round win for the weekend and wisely settled for second.
``I was happy with my performance in that race. I learnt a lot this weekend about my race management. It was frustrating to not finish the first race but it was 100% my fault,'' Duncan said.
At the halfway point in the championship, one point separates Fontanesi (115) and Lancelot (114). Duncan (108) is breathing down both their necks.
``We are still in the championship hunt. We are fast enough but I just have to be smarter with my racecraft,'' Duncan said.
Duncan's coach Josh Coppins believes she can turn the championship around if she can put the mistakes behind her.
``Hopefully, she can start to iron them out which she did [when she won race 2]. Everything else ran smoothly, including the bike and she rode well,'' Coppins said.
Duncan will remain in Europe at her base in Belgium and prepare for the next WMX round, in the Czech Republic on July 22-23.
