Duncan beat off competition from Andreas Matt, Daniel Bohnacker and fellow Canadian Brady Leman in the big final in Italy to record a second win in the space of 24 hours.
And that saw him jump up all the way to the top of the overall ski cross World Cup standings, Duncan leading the way by nine points ahead of Matt with Brady third, 44 behind in total.
Elsewhere in the second of the women's events, Katrin Mueller claimed her second win in three races but second for Marielle Thompson put her into the overall lead.
Mueller led home Thompson in the big final before Heidi Zacher in third and Anna Woerner in fourth with Saturday's winner Fanny Smith second in the small final for sixth overall.
Thompson now leads the women's ski cross World Cup standings by 14 points ahead of Mueller and Smith, who are tied in second place overall with 256 points.
© Sportsbeat 2013