Duncan's bullying does not excuse Harold in the slightest, merely because it was ultimately a conscious decision made by Harold out of spite. Harold is ultimately responsible for his own actions: he is the one who made them.
Simple logic disctates that Harold is responsible for his own actions. Duncan and his pranks are irrelevant because Harold cheated off Courtney, not him. He just wanted Duncan to suffer. There is no excuse for such an act done purely of malice. Say what you will of Courtney's harshness, she was willing to help Harold out (giving him the strategy to win in dodgeball, and actually letting him go in the talent contest. Please recall that Geoff, DJ, and Courtney all had excellent talents lined up. A little harsh, yes, but not malevolent.)
Harold, by contrast, doesn't have anything to balance out his cheating. He's willing to work with the team for the challenges, but we rarely, if ever, see kindness out of him.