Duncan was born at Twickenham on 20 April 1821, his father, Peter King-Duncan, a descendant of an old Scottish family, being a leather merchant; his mother was daughter of Captain R. Martin, R.N., of Ilford, Essex.
He received his earlier education first at the grammar school, Twickenham, next at Nyon, by the lake of Geneva, after which he was apprenticed in 1840 to a medical practitioner in London.
[1] In 1842, Duncan entered on the medical side at King's College, London, passing through it with distinction, and being elected an associate in 1849, after graduating as M.B.
[1] His separate scientific papers are not less than a hundred in number, and his 'Supplement' to the Tertiary and Secondary Corals forms a volume in the publications of the Palaeontographical Society.
[1] He made a special study of the corals and echinids, taking also much interest in the ophiurids, sponges, and protozoa, regarding all questions from the point of view not only of the philosophical zoologist, but also of one who applied the distribution of species to elucidate ancient physical geography.
He described the fossil coral fauna of Malta, Java, Hindustan, Australia, Tasmania, and the West Indies, the echinids of Sind, and of other countries.