In 2020 the First Sea Lord described Duncan as “one of the most influential naval officers of the last 50 years”. Following studies as an opera singer at the Royal College of Music and Royal Birmingham Conservatoire, Duncan began his career as a naval rating. He was quickly promoted to officer. Having assumed the duties from someone three ranks senior at no notice and leading his new department during operational sea training he was awarded a commendation. He became Admiral’s Secretary to the Flag Officer Plymouth where he reorganised the Headquarters creating the model for all military HQs. He instigated the initial studies which led to women enjoying full career opportunities in the Armed Forces and also led major changes after investigating racism in the Armed Forces. He served in many trouble spots ranging from the Falklands to Northern Ireland and received a third commendation for his work as a Group Logistics Officer in the first Gulf war. 

In 1994 Duncan was about to become Military Assistant to the Prime Minister when he was approached by a blackmailer. Duncan immediately reported this to the head of the Special Investigation Branch of the Naval Police. He was arrested for being gay and endured two days of interrogation about his private life. He was dismissed six months later. Duncan became an adviser to Rank Outsiders, a small group of ex service personnel who provided welfare and advice to those being investigated because of their sexuality. He led the campaign to end the ban on ‘gays in the military’ and was a test case at the European Court of Human Rights. He led the evidence given to the Defence Select Committee and was a regular contributor to the national and international debate. 

Following his dismissal Duncan worked as Deputy Director and Finance Director of the Human Rights group Liberty. He was then CEO of the largest firm of Human Rights Solicitors in the UK before becoming Deputy Director General of the Advertising Standards Authority. When the European Court of Human Rights ruled in his favour Duncan returned to sea commanding specialist ships such as icebreakers, survey ships and ocean-going salvage tugs and was promoted to Commodore in 2003.

He was instrumental in changing the law on three occasions, in 2000 lifting the ban on lesbians and gays in the military throughout Europe, in 2016 the removal of legislation which discriminated against the lesbian and gay community in the Criminal Justice Acts and in 2017 the removal of legislation discriminating against lesbians and gay people in the Merchant Navy Acts. 

He advised the campaign in Trinidad and Tobago to repeal anti LGBTQ laws and assisted the campaigns for USA LGBT people. He advised the successful campaign to return medals to LGBTQ+ personnel. 

In 2020 he persuaded English Heritage to review and amend their procedures which inadvertently discriminated against ethnic minority communities in the provision of blue plaques on buildings. Several new plaques for those communities were subsequently approved that year.

In 2022 he finally persuaded the government to commence an official inquiry into the way LGBTQ+ people had been treated by the military under the old ban and was an adviser to Lord Etherton who chaired the Inquiry. This led to restitution being given to the victims of the policy. Duncan is Chair of the largest Fringe festival in England, Brighton Fringe. He and his husband, JL, live in Brighton.