Duncan grew up Fort Frances, Ontario. As a youngster, he idolized NHL stars such as Ray Bourque, Cam Neely and Wayne Gretzky and his favorite team was the Boston Bruins.
Playing youth hockey in Fort Frances Ontario at the age of 8 Duncan’s first team was in the Tom Thumb league playing forward for the Fort Frances Times Tigers. At the age of 9 he tried out for the Pinewood Sports Atom Rep team and made the team again playing forward. In his second year, at 10 years old, his coaches Gib and Bill Tucker decided that Duncan was better suited at defense. He switched from forward to defenceman and has not looked back since. Duncan played his PeeWee years with the Knights of Columbus. At the age of 14, Keith moved to Penticton British Columbia where he finished out his last two years of minor hockey.