On 10 March 1973, having attended Royal Military Academy Sandhurst, Duncan was commissioned into The Prince of Wales's Own Regiment of Yorkshire, British Army, as a second lieutenant with seniority from 8 March 1973. Duncan eventually rose to command the regiment in 1990 as a lieutenant-colonel, and would lead the PWO through two deployments. The first was to Northern Ireland in 1992 to fight in the Troubles, with Duncan appointed an Officer in the Order of the British Empire for his performance in command. The second was in 1993 to Bosnia as part of UNPFOR. It was here that Major-General Duncan was awarded the Distinguished Service Order for his leadership in action, with the Telegraph calling him "instrumental in the rescue of 200 Croats...". After the tour he was commissioned as a commander of the 19th Mechanised Brigade in Germany, and then took up a position in the Ministry of Defence as Director of Land Warfare. In 2000 he was appointed Chief of Staff of the United Nations Mission in Sierra Leone. UNAMSIL would eventually result in the disarming of more than 75,000 fighters. It rebuilt the country's police force and paved the way to democratic elections. Though stepping down from this position in 2001 to take up his final role in the military as Director-General of Training Support, the United Nations mission was completed in 2005, and Duncan was invested as a Commander of the Order of the British Empire for his efforts. In 2005, Major-General Duncan retired from the army after 43 years of service.
Duncan was married three times and had three children. His first wife was Anita Keily, and together they had two sons; Thomas and Edward. They divorced in 1993. In 1995, he married Avril Walker, and together they had a daughter, Arabella. They divorced in 2008. In September 2013, he married for a third time to Ellen Le Brun, and she survives him.
Health
In 1993, during his posting to Bosnia, Ducan suffered a brain injury when the Warrior tracked armoured vehicle he was travelling in was damaged by a roadside bomb. He subsequently developed post-traumatic stress disorder in relation to the incident. In 1999, he took mefloquine, an anti-malarial drug, for six months in preparation for his deployment to Sierra Leone. This drug was blamed for his later mental health problems and the worsening of his PTSD.