Duncan is a Consultant Respiratory Physician who worked in East London and was the Medical Director of Barts and The London NHS Trust. He became an Associate Director of the Performance Development Team at the NHS Modernisation Agency and later Dean of the Bedfordshire and Hertfordshire Postgraduate Medical School. More recently, he has served as a Trustee of the British Lung Foundation. He has published papers on respiratory medicine and medical management topics and was Editor of the British Journal of Diseases of the Chest. His youngest daughter is a practising osteopath.