In 1789 Duncker obtained the rank of 2:nd Lieutenant in the Savolax ranger regiment. He fought in the 1790 war against Russia and proved his valor at the Battle of Perttimäki May 19. In 1804 Duncker was promoted to Captain.
During the Finnish War of 1808-1809 he distinguished himself as a brave and prominent officer. When Cronstedt's army retreated from Mikkeli to Iisalmi and Oulu through Leppävirta (March 1808), Duncker commanded the army's rearguard. During the Battle of Pulkkila (May 2, 1808) he distinguished himself so well that Johan August Sandels gave him the honorary assignment to bring the news of the victory to the Swedish king. Shortly after, he was promoted to Major. In June 1808 he captured a large transport of suplies to the Russians. During the Battle of Virta Bridge (October 27 1808), Duncker together with Colonel Fahlander and Major Malm and only 600 men helped Sandels to utterly defeated a superior Russian force. In 1809 Duncker fought the Russians in the Swedish province of Westrobothnia, and was promoted to Lieutenant-Colonel. During the Battle of Hörnefors (July 5, 1809) Duncker commanded the Swedish rearguard and received fatal wounds and died in the Russian encampment later that day.
Duncker was buried in Umeå cemetery by the Russians that gave him a full honour guard.