In Scottish the meaning of the name Duncan is: From the Irish name Donnchadh, meaning brown warrior.
I've not found any odd spellings in my Grandmothers Duncan line so far but on doing some research into names I have realised there may be more options out there to navigate than I first thought.
Some variations on Ancestry.co.uk - Duncan, Duncam, Duncain, Duncane, Duncaime, Duncambe, Duncana, Duncand, Duncen, Duncem, Dunccan, Dunccn, Duncaw, Duncean, Dunchan, Duncham, Dunchin, Duncian, Duncin, Dunckan, Duncken, Duncn, Duncom, Duncomb, Dunbome, Duncomd, Duncome, Duncone, Duncoomb, Duncoon, Duncum, Duncumbe, Duncun,