Educated at, amongst other places, George Heriot's School, Edinburgh, Duncan has made his career in the Valuation Office Agency. He qualified as a Chartered Surveyor in 1981 and has spent all his adult life in East Anglia, working in Colchester, Bury St Edmunds, Norwich and Cambridge.  

Between 2001 and 2006 Duncan was head of Rating and Council Tax at the VOA in Norwich before moving to the same role based in the Cambridge location. Since 2011 he has worked as a Technical Advisor in the VOA, and currently works with RVU East as well as providing advice nationally on council tax and exemption matters.

In 1997 he commenced study for the RICS special Rating Diploma and passed in 1999 being awarded the Machinery Users Association prize. In 2011 he was chairman of the Rating Diploma Holders' Section of the RICS and from 2013 until 2020 he was the Course Director for the new style Rating Diploma Study Course. He has also been an APC assessor for the RICS and is a member of the RICS  Local Taxation and Policy Panel. In June 2019 Duncan was awarded an MBE for services to the surveying profession.

His knowledge of rating and rating law is formidable. He is considered to be a tough advocate and expert witness in the courts. He has lectured widely on rating and local taxation matters and his lively and interactive seminars are always well appreciated.