Duncan studied at the Royal College of Music and subsequently offered the position of Trainee Répétiteur at English National Opera under the tutelage of Sir Mark Elder. He then became a full-time member of the music staff of Opera North and again at English National Opera. Since 1997 he has been working freelance for all the major international festivals including Salzburg and Glyndebourne. Duncan has had a close relationship with the latter where he specialises in 19th and 20th century German Czech repertoire.
Duncan has collaborated with numerous internationally renowned conductors including Robin Ticciati, Sir Simon Rattle, Sir Mark Elder, Kazushi Ono, Jiří Bělohlávek, Sir Charles Mackerras and Vladimir Jurowski. He held a teaching position at the Royal Conservatoire of Scotland for many years and is in demand for auditions for all the major opera houses. Duncan lives in Scotland.