Duffy's three RBIs matched the Giants' total runs scored in the previous four games on the homestand entering Tuesday.
The rookie got the Giants on the board in the second inning, crushing a J.A. Happ fastball on a 2-2 count into the left-field bleachers. Duffy then hit a soft fly ball that dropped between center fielder James Jones and second baseman Robinson Cano to extend the Giants' lead in the eighth.
"He's a ballplayer," Giants manager Bruce Bochy said.