Duncan went to Paris, where he became acquainted with the minister Jean-Baptiste Colbert, by whom he was appointed physician-general to the army before Saint-Omer, commanded by the Duke of Orléans in 1677. After the peace of Nimeguen, he left the army, published in Paris his first medical work in 1678, and then passed two years in London, where he employed himself especially in collecting information about the great plague of 1666. In 1681 he was summoned back to Paris to attend his patron Colbert, after whose death in 1683 he returned to his native town of Montauban.