Rubuta ci gaba don wannan sakin layi - An haife shi a Ayrshire, Duncan ya halarci makarantar Ardrossan a Ardrossan, Arewacin Ayrshire kuma ya yi karatu a Jami'ar Birmingham inda ya sami BCom a 1985. Daga baya ya yi aiki a matsayin mai ba da shawara kan harkokin kasuwanci da sadarwa ga Mackays Stores Ltd daga 1985 zuwa 1988 ban da gudanar da kasuwancin iyali - John Duncan & Son daga 1988 zuwa 2000. A watan Satumbar 1997, ya bayyana a matsayin memba na masu sauraro yayin muhawarar Talabijin ta Scotland game da raba gardama
, kuma ya yi magana game da kafa majalisar Scottish. Daga baya Duncan ya tsaya takara a zaben majalisar da ya fara adawa da ita.