Duncan hit a three-run drive off starter Adam Eaton in the first and added a two-run double off Chad Durbin in the third.
"Most of the time, especially at the start, you don't even worry about the results," Duncan said. "You don't make too much out of it. You worry about the process, and getting to the point where you're comfortable and consistent."
Duncan is trying to earn a regular role at first base, where the crowded competition includes part-time DH Jason Giambi, Wilson Betemit and Morgan Ensberg.
"Shelley is going to give you everything he's got every time he's out there. He's shown he can compete at a high level," Yankees manager Joe Girardi said.
Chien-Ming Wang allowed one hit in two scoreless innings for New York and is lined up to start the season opener March 31 against Toronto. His only mistake was a second-inning double by Ryan Howard.
"Today my changeup was good, the slider was working and I got just one pitch to Howard up," Wang said.
Betemit also homered for the Yankees. Derek Jeter and Bobby Abreu each had a stolen base.
Duncan's homer spoiled Eaton's spring training debut. The right-hander is trying to secure the fifth spot in the rotation after going 10-10 with a 6.29 ERA in the first year of a $24.5 million, three-year deal last season.
"Three pitches in a row - single, single, homer. Only one of those was where I wanted it, and it wasn't the home run pitch," said Eaton, who appears to be competing with Durbin for the No. 5 spot. "I thought I was going out there for the third inning, but (pitching coach Rich Dubee) said it was enough, and I came in to work out."
Durbin, a free-agent acquisition this offseason, didn't fare too well, either. He allowed two runs and three hits in two innings compared with Eaton's three runs and five hits in two innings.
"I think that's more for management side of things, more for (the media) and more for the fans and stuff like that," Durbin said about the competition. "We're trying to get stuff where we need to be and not worrying about those outside variables right now."
