Duncan was born on 23 June 1961 and grew up on his parents' farm in Limuru. He began driving around the age of ten, assisting his mother when her vehicle became stuck on farm roads during the rainy season. While attending St. Mary's High School, he developed a primary interest in the mechanical repair of cars and motorcycles. Before transitioning to rallying, he competed in motocross, where he secured the national 125 cc title in both 1979 and 1980.
Duncan made his debut in the Safari Rally in 1983. Driving a Nissan pick-up and navigated by Gavin Bennett, he finished in ninth position and secured a class victory. This performance brought him to the attention of the rallying community. His first victory in a national rally occurred at the 1987 Nakuru Rally, where he drove a Toyota Celica twin-cam turbo navigated by Ian Munro. That same year, he won the majority of his races to claim his first national championship and was named the 1987 national Motor Sportsman of the Year.
During the 1990s, Duncan campaigned Group A machinery, including the Subaru Legacy RS and the Toyota Celica Turbo 4WD. In 1990, he competed in the Acropolis Rally in Greece with navigator Yvonne Mehta; he finished eighth overall and won the Group N class in a Subaru Legacy RS.
His most significant international success came in his home WRC event, the Safari Rally. In 1993, he finished third overall as part of a Toyota 1-2-3-4 sweep. In 1994, he achieved his career-best result by winning the 42nd Trustbank Safari Rally outright.
In later years, Duncan competed in the Kenyan National Rally Championship (KNRC) using a non-homologated Toyota Hilux equipped with a turbocharged 4.5 L engine. Despite a four percent time penalty imposed due to the vehicle's non-homologated status, he won the 2006 Guru Nanak Rally. This victory made him the first driver to win a single KNRC round ten times, a record he extended by winning the same event again in 2007.
In 2011, Duncan secured his sixth Kenyan National Rally Championship title. The following year, he narrowly missed a second victory in the Safari Rally—then held under African Rally Championship (ARC) status—finishing 15 seconds behind winner Carl Tundo.
Duncan has maintained a diverse competitive career across multiple disciplines. In 1998, 2006, and 2007, he won the Rhino Charge off-road event. In 2003, he demonstrated his versatility by finishing as the runner-up in the Kenyan national championships for enduro, motocross, and rallying simultaneously.
He won the national autocross championship in 2008 and introduced a Nissan Patrol pick-up for rally competition that October. In 2009, driving a 1968 Ford Mustang navigated by Amaar Slatch, Duncan won the Safari Classic Rally, defeating defending champion Björn Waldegård. He returned to the Classic Safari Rally in 2011 in a Ford Capri, finishing in seventh place.
Duncan's championship titles in the Kenyan National Rally Championship span twenty-four years, with overall wins in 1987, 1988, 1989, 1991, 2000, and 2011. His 1994 Safari Rally win remains a rare instance of a Kenyan driver taking an outright victory in a World Rally Championship round.