Dunn finished with three hits and four RBI in support of Craig Stammen (4-4), who gave up just one run on six hits with six strikeouts and four walks over 5 1/3 frames. Adam Kennedy had two hits and two runs batted in, while Roger Bernadina finished 2-for-4 with two runs scored. The Nationals have won two of three games.
Kelly Johnson finished 3-for-5 and Adam LaRoche drove in a run for Arizona, losers in two of three. Ian Kennedy (6-9) took the loss, yielding four runs on eight hits with two strikeouts over just four frames.
"I didn't think I had a particularly good game myself tonight," Arizona interim manager Kirk Gibson said. "I made some mistakes. I wasn't really on top of it."
Arizona slugger Mark Reynolds left the game during a pitching change in the top of the seventh frame after appearing to look disoriented. He was struck in the head by a pitch in Tuesday's game, yet did pass a neurological exam after the contest.
The Nationals used Dunn's power in the first, as the slugging first baseman clubbed a two-run shot inside the foul pole in right.
"I knew it was going to be close," Dunn said. "I didn't know if it was going to be fair or foul."
Arizona came back with a run in the home half on Justin Upton's sacrifice fly, but it was the only offensive output for the home team until LaRoche's solo shot in the eighth.
Bernadina singled, stole second and scored on Kennedy's single in the third, while Arizona lost out on a first-and-third, one-out chance in the home half when Miguel Montero grounded into a double play.
Dunn's second long ball of the game again came to right field in the fourth, and the combination of Bernadina and Kennedy were at it again in the seventh. Bernadian singled in front of Kennedy's two-bagger then Dunn's base hit to right plated Kennedy for a 6-1 game.
Michael Morse's solo home run in the top of the eighth made the score 7-1.
Wednesday's game marked Stammen's first career start versus Arizona...The Nationals have won seven of their last nine meetings with the Diamondbacks...Each team grounded into three double plays on the game...Dunn recorded his 28th career multi-homer game.