Unbeatable Johaug headlines Norwegian domination in Östersund
Another Norwegian, Ingvild Flugstad Oestberg, was a further 1.7s back to claim bronze while home favourite Ebba Andersson was the only non-Norwegian in the top six as she came fourth.
Johaug's lead in the overall World Cup standings becomes more unassailable with each passing race as she moved on to 1,740 points – 502 clear of Russia's Natalya Nepryayeva, who struggled to tenth in Ostersund, with Weng a further 70 points adrift.
In the men's event, Sjur Roethe headed another Norwegian clean sweep with Simen Hegstad Krueger and Finn Haagen Krogh completing the podium.
Roethe's time of 30:41.5mins was good enough to triumph in the 15km freestyle event and notch a first World Cup win of the season.
Krueger finished just 1.7s behind, with Krogh a further 14.9s back and the ageless Martin Johnsrud Sundby making it an all-Norwegian top four.
Overall World Cup leader Alexander Bolshunov came fifth on the day to maintain his dominant advantage atop the season-long standings.