The Avalanche scored three more goals against the Senators than they had in their first three games combined.
Colorado produced three power-play goals in the final period and four overall, making things easy for goaltender Jean-Sebastien Giguere. He stopped 15-of-16 shots while Colorado was winning its third straight.
Joakim Lindstrom scored his first two goals of the season, producing the only tally of the second period to give the Avalanche a 3-1 lead and then finishing the onslaught with 2:33 to go in the third.
Duchene's goal came 1:48 into the third. Daniel Winnik scored shorthanded less than 3 minutes later, after which David Jones and Lindstrom tallied on the power play.
All seven goals came against Craig Anderson on 39 shots. The Senators have opened the season with three losses in four games.
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'
Beyonce flaunts bikini body, Blue Ivy in vacation pics