The United States-born Fiva, who was eliminated in the first round at the Sochi Olympics, won the final ahead of Tomas Kraus of the Czech Republic, Didrik Bastian Juell of Norway and Christopher Del Bosco of Montreal.
Duncan, from London, Ont., won the small final and is clinging to a tight lead in the standings as he aims to become the first Canadian to win a men's skicross Crystal Globe.
He has 361 points, nine more than Austria's Andreas Matt with three races to go.
"It's been a see-saw battle all year. I'm happy to have the red bib back," Duncan said of the bib given to the World Cup leader. "Everything is so tight and there are 300 points up for grabs still."
Duncan finished 26th at the Sochi Games last month.
"It would be incredible to take home an overall title. It would make it a lot easier to forget the Olympics," he joked. "I'm happy with fifth but I felt I was capable of a bit more today."
Calgary's Brady Leman was right behind Duncan in sixth and Louis-Pierre Helie of Berthierville, Que., was 11th. Mathieu Leduc of Comox, B.C., was 18th and Tristan Tafel of Canmore, Alta., was 24th.
Gjefsen edged Swiss racer Fanny Smith, the 2013 world champion, with Ophelie David of France third and Anna Holmlund of Sweden fourth.
Thompson, the reigning Olympic champion from Whistler, B.C., won the small final. She has a 75-point lead on Smith in the standings.
"Now that the Olympics are over I'm focusing on being consistent and hopefully I can end the season strong and get the (overall)," said Thompson, who went 1-2 in the Olympic final with her teammate Kelsey Serwa, who has taken the rest of the season off.
"Winning the Crystal Globe would be a great topper to the season and a reward for my consistency and all the work I put in during the off-season."
Danielle Sundquist of Calgary was 13th.
With files from The Canadian Press.