Duffield grew up in Harrisburg, Pennsylvania with his parents, Brian and Brenda Duffield, and two sisters. In 1995, at nine years old, he moved to Ireland when his parents decided to become missionaries. He described the surroundings as completely remote and lagging in popular culture. Duffield attended school where in one classroom, three grades were being taught by one teacher, which he described as very "Little House on the Prairie-esque". He moved back to the US when he graduated high school in 2004.
Duffield graduated from both Temple University and Messiah College in 2008. After graduation, he moved to Universal City, California for a summer internship through Temple University at the Oakwoods apartments prior to the 2007–08 Writers Guild of America strike and at the beginning of the Great Recession.
From there, Duffield worked various jobs before becoming a professional screenwriter. His first job in Los Angeles was at the now defunct Storyopolis, a children's book store/art gallery, in Studio City, where he worked with writer Blake Harris. He has also worked as an assistant and script reader within the film industry for several years. Prior to selling his first spec screenplay, Duffield was temping at the Lucky Brand Jeans factory in nearby Vernon, California.