Duncan had been treated for tuberculosis in 1900. Childless, she died of chronic lung disease on 22 July 1922 at Ashtead, Surrey, whence she and her husband had moved in 1921. She had been a smoker and it is possible that the cause of death was emphysema, although her lung problems generally may have been exacerbated by the climate and sanitation in Calcutta. She was buried at St Giles's Church, Ashtead, and left a CAD$13,000 estate. Though she rarely returned to Canada after marrying Cotes, and last visited in 1919, she had always insisted that the royalties from her books were paid into her bank account in Brantford. Everard Cotes, who was her beneficiary and worked as parliamentary correspondent for the Christian Science Monitor, outlived her and remarried in 1923, fathering two children before his death in 1944.