Duncan had a hard past, he's moved from all over living in Highever, moving to Ferelden to Orlais. After the tragic death of his parents Duncan moved Val Royeaux were he learnt the trait of being a theif, which was the only way to survive on the streets.
Not much older than a boy, Duncan was signed into the order by Commander Genevieve, which was a bit of luck because during on that morning he was about to be executed for murder.
Later that day he got informed the boy he murder was a Grey Warden, he got offered a place to join among The Warden but Duncan refused as he thought the life of a Warden must be horrible. He wasn't all wrong at first, Duncan fellow companions despised him and everything he stoof for. He attempted he to run away numerous of times and he never left his theiving days behind as when the opportunity came, he would steal.
Later he life Duncan found himself as the Warden Commander, a job that entitles of recruiting others into The Warden life. He did recruit The Warden and Alistair beofre the battle at Ostagar.