Kammala jumla mai zuwa: An haifi Duncan kuma ya girma a ƙauyen Chanceford Township a kudu maso gabashin York County, Pennsylvania, wanda ke iyaka da babban kogin Susquehanna. An nada shi zuwa Jami'ar Sojojin Amurka a ranar 1 ga Yuli, 1845, kuma ya kammala karatu daga West Point a watan Yunin 1849. Wani dalibi mai kyau, ya kasance na biyar a cikin ɗalibai 43. Tare da matsayinsa na ilimi, an ba shi izini a matsayin na biyu kuma an sanya shi zuwa Artillery na biyu na Amurka a watan Yuli na wannan shekarar. Ba da daɗewa ba bayan haka, a ranar 31 ga Oktoba, an ba shi cikakken matsayi na laftanar na biyu kuma an sake sanya shi zuwa na Uku na Artillery. Ya ga aikinsa na farko na yaƙi yana aiki a
Rikicin Florida da 'yan asalin Seminole.