Duncan qualified in 2003 and became a partner  in 2011. He is a Law Society of Scotland Accredited Specialist in Medical Negligence (Defender) and is recommended by both Chambers & Legal 500 in that discipline. He has been described as "very experienced" and "highly regarded." Also, Duncan has been ranked in Professional Discipline by Chambers and has been praised by interviewees for his "client management and knowledge of the medical-legal aspects of the law." Others add: "He has a certain flair about him; everything's done efficiently and everything is succinct and well thought out."

In addition to court work Duncan also appears before various healthcare regulators including the Medical Practitioners Tribunal Service, General Dental Council and also the Health & Care Professions Council.  He is a member of the Association of Regulatory and Disciplinary Lawyers and is recommended by Chambers Directory for his professional discipline expertise. His clients include medical and dental professionals, their unions and insurers.

Duncan defends medical product actions including trans-vaginal mesh and metal on metal hip implant cases. He also provides his professional negligence expertise to a variety of professions including accountants, insurance brokers and IFAs.