Duncan was born in Batesburg, South Carolina and graduated from high school from W.W. Wyman King Academy. He earned a degree in journalism from the University of South Carolina and worked for seven years at the Greensboro News & Record. His novelette "Close Encounters" won the 2012 Nebula Award for Best Novelette.
Duncan earned an M.A. in creative writing (fiction) from North Carolina State University and an M.F.A. in fiction writing from the University of Alabama. He also attended Clarion West Writers Workshop in 1994.
In Fall 2008, he was hired as an Assistant Professor of English at Frostburg State University in Frostburg, MD.
His fiction has appeared in a number of venues, including Asimov's Science Fiction, Realms of Fantasy, Weird Tales, SciFiction, and Escape Pod. He has also published poetry, essays, and reviews.
Duncan currently lives with his wife Sydney in Frostburg, Maryland along with remarkable 17 year old dog Lily, and cats Bella and Hilary.
He was a senior editor at Overdrive, a magazine for truck drivers, from 2003 to 2008.
Duncan was an instructor at Clarion Workshop in 2004 and at Clarion West in 2005.
He has frequently given readings and spoken on panels at such venues as the International Conference on the Fantastic in the Arts, held each spring in Florida.
Duncan starred as the main character, Counter, in a live dramatization of Jeanne Beckwith's one-act play The Back Room, performed with award-winning authors John Kessel and James Morrow, author and scholar Dr. F. Brett Cox, writer and critic Fiona Kelleghan, Sydney Sowers, and Buffy the Vampire Slayer expert Dr. Rhonda V. Wilcox. The play was presented at the 17th International Conference on the Fantastic in the Arts, held in Fort Lauderdale, Florida, March 19, 1999.
He has won the Theodore Sturgeon Award. and three World Fantasy Awards, and has been nominated for Hugo, Nebula Award and Shirley Jackson Award. The Night Cache was nominated in the Best Novella category for a 2010 World Fantasy Award.
He won the 2012 Nebula Award for Best Novelette for "Close Encounters" featured in The Pottawatomie Giant and Other Stories.