Duncan grew up on his parents' farm in Limuru, Kenya. As a child he developed mechanical aptitude early, helping his mother navigate muddy farm roads and spending his school years at St. Mary's High School more interested in repairing cars and motorcycles than academic study. Before turning to four wheels he raced motocross, winning the national 125 cc title in 1979 and 1980.
Duncan entered the Safari Rally for the first time in 1983, co-driven by Gavin Bennett in a Nissan pick-up, finishing ninth overall and taking a class win. The result drew attention from the rally community and set the foundation for what would become a career defined by the Safari.
His first national championship came in 1987, the same year he took the Nakuru Rally driving a Toyota Celica twin-cam turbo navigated by Ian Munro. That season also earned him Kenya's Motor Sportsman of the Year Award. He went on to claim the Kenyan National Rally Championship in 1988, 1989, 1991, 2000, and finally in 2011 — a span of 24 years between first and sixth titles.
On the world stage, Duncan's most celebrated result came at the 1994 Safari Rally, the 42nd running of the event, which he won outright. That victory was part of seven consecutive top-ten finishes in the Safari from 1990 to 1996, a record of consistency that stands out in an event where mechanical attrition can eliminate the majority of the field. His best result outside the Safari on the WRC calendar was third overall at the 1993 Safari Rally, during which Toyota achieved a 1-2-3-4 finish. He also finished eighth overall at the 1990 Acropolis Rally in Greece, co-driven by Yvonne Mehta, taking the Group N class win in a Subaru Legacy RS.
Duncan competed with a Group A Subaru Legacy RS and Toyota Celica Turbo 4WD through the 1990s, though his WRC appearances beyond the Safari were limited.
After the 1990s Duncan continued competing in the Kenyan National Rally Championship, latterly in a non-homologated Toyota Hilux fitted with a turbocharged 4.5-litre engine. In November 2006 he won the Guru Nanak Rally for the tenth time — the first driver in KNRC history to reach that milestone — and extended the record with an eleventh victory at the same event in 2007, despite receiving a four-percent time penalty for his non-homologated vehicle.
Duncan also excelled in off-road competition. He won the demanding Rhino Charge event in 1998, 2006, and 2007. In 2009, fifteen years after his Safari Rally win, he claimed the Safari Classic Rally driving a 1968 Ford Mustang co-driven by Amaar Slatch, with Safari legend Björn Waldegård finishing second. He defended the Classic title into 2011 driving a Ford Capri.
In 2012 he narrowly missed a second outright Safari Rally victory, by then running under African Rally Championship status, losing to Carl Tundo by just 15 seconds.
Duncan's career spans more than three decades of competitive driving across rally, off-road, motocross, and autocross disciplines. His six national titles, his 1994 Safari outright win, and seven consecutive top-ten finishes in arguably the world's most attrition-heavy rally cement his place among the defining figures of Kenyan and East African motorsport.