Duncan was 13 shots off the pace at the halfway stage of The 55th Open, but a spectacular turnaround in his fortunes followed, together with a dramatic collapse from leader Abe Mitchell.
The first day of the Championship could hardly have gone better for Mitchell. After shooting 74 in the morning, he opened up a commanding six-stroke lead with an afternoon 73 that featured a hole-in-one at Royal Cinque Ports' eighth hole.
At that point, Jim Barnes and Sandy Herd were Mitchell's nearest challengers, while Duncan languished outside of the top 10 following two successive rounds of 80.
Crucially, however, Duncan ended a disappointing day by examining the equipment that had been placed on sale in the Exhibition Tent at the course.
The Scot opted to purchase a driver and, with the new club added to his bag, he promptly shot 71 the following morning to charge up the leaderboard.
News of Duncan's excellent performance reached Mitchell just before the latter began his third round and perhaps unnerved the runaway leader.
Mitchell slumped to a painful 84, meaning he suddenly shared fourth place with Duncan - a player he had led by 13 at the start of the day.
The pair trailed Barnes and Herd, who each carded 77s, by one, while Len Holland was a further shot ahead after matching Duncan's 71.
Holland was then the next frontrunner to buckle as he carded a 79 in the final round and Duncan took full advantage.
A fine 72 was enough to secure a victory that had seemed hugely unlikely only 24 hours earlier. He triumphed by two shots from Herd, with Ted Ray third and Mitchell - who had been in control after 36 holes - four shots back in fourth.
More than 100 years on from his dramatic success, Duncan remains the last man to win a Major Championship with a round in the 80s.