Duncan won the Yorkshire Cup thanks to a clever tactical ride from William Buick for trainer John Gosden.
The four-year-old travelled wide of the pack early on before being brought back to the field, and Buick then sent the 11-4 shot to the far rail where he was able to stay clear of his rivals.
Blue Bajan (25-1), trained by David O'Meara and ridden by Daniel Tudhope, briefly threatened but came second.
Luca Cumani's Manighar (7-1) finished third in the eight-strong field.
Duncan was giving weight to all of his rivals, but Buick - under a planned strategy - was happy to let his mount, who had been gelded over the winner, to run freely on ground uncharted by the other horses.
It was hard to tell whether the tactic would work early on, but although the first three horses home were covered by a single length, Duncan was good value for his win.
The victory puts Gosden on the map in the British Champions Series, with this 1m 6f race the third race of the season to feature in the new calendar of big Flat races.
Gosden said: "He's a bit of a character but he has a ton of talent. It's been a little difficult to get it out of him at times but he's got in a bit of a groove more recently.
"I expected him to run a huge race today. He was a little effervescent and fresh and I'm sure with that out of his system he'll be able to go further.
"William said he had trouble pulling up. We'll give the Ascot Gold Cup some thought. He's got a turn of foot but he needs to start switching off a bit more.
"We'll give him every chance to show us what he wants to do. He won't be running in a hurry that's for sure."
Duncan was cut to 10-1 from 14s for the Ascot Gold Cup by one leading bookmaker.