Duncan will never be redeemed. His relationship with Gwen was formed through deceit and cheating. Scott was injured and he changed his way (although Scott has some antagonism in him). Duncan remains a bad guy, who learns zero responsibility about his actions. Take Courtney's Island elimination. Duncan claims that Harold cheated off Courtney, but he had a reason. Duncan was bullying Harold, so he took his anger on Courtney (to see how Duncan would react).