Duncan's younger brother, Ewen was also serving in India at the time of the Lucknow Relief operation as a Lieutenant in the 93rd Regiment. Cromb's Highland Regiments tells us that just before action commenced, Duncan met Ewen and gave him his ring and some other trinkets he was wearing saying "Here, Ewen, take these. If I come out of this all right I'll get them back from you; if I don't, they are yours." Of course he survived as did Ewen.
Duncan next served in the Rohilcund campaign in the Punjab the following year. He was promoted to Major in 1865 and commanded a battalion in actions at Amoaful and Coomassie. During the former action he was wounded twice but refused to go to the rear; rather he supported himself on a stick and lead his men in the final assault. For his valour, he was promoted to Brevet-Colonel and received the Companionship of the Bath.
Duncan was later made commander of the Black Watch and during the action at Tel-el-Kebir in 1882, he led his regiment over the enemy trenches. Brother Ewen also served in that action as commander of the 93rd. Duncan's final assignment was commander of the Regimental District at Perth.
Duncan retired from military service in 1885 when he succeeded his father as Chief, a position he held for just 21 months before his death at Cluny Castle on 3rd October 1886. He was married to Emmeline , daughter of Major-General Harris of the Bengal Army but they had no children.