I threw this together in an hour because I actively hate being a fan of both of these characters sometimes. The nuances of them are often just pushed aside for the sake of 'Harold deserved the bullying' or 'Duncan is a one-note jerk'.
Duncan and Harold are great rivals because they have a lot in common beneath the surface, and often times are these similarities contrasting with one another. Duncan is predominantly cool, quippy, yet has a hidden nerdy, dorkish side that he would rather die than show. Harold is a geeky loser who plans to live in his mom's basement, yet has a select few moments his bite matches his bark. Even in design, they both wear undershirts while having facial hair, with Duncan's being well-kempt while Harold's isn't.
What's doubly interesting is how their main connecting trait is their arrogance. Duncan backs it up more often, but they're both exceedingly arrogant people who often put their own foot in their mouth. They both have a sense of 'I'm so awesome' and it's why they're often used as punching bags for the show's slapstick. They aren't perfect, pristine people and are notably flawed, yet also aren't quite the level of characters like Alejandro or Heather. Even lesser villains like Justin and Scott are arguably worse people than these two, because despite Duncan and Harold both being arrogant; they're also genuinely good people at heart who like to help others.
Harold is the only person to ever reach out and help Heather. This might've been due to his main character sydrome, but he went out of his way to help a fellow outcast just because he didn't like seeing her sad. Similarly, Duncan goes out of his way to get a new pet Bunnie for DJ, simply because he doesn't want his friend to go through the same thing. Harold often finds the best in bad people, while Duncan often socializes with exceedingly good people.
Even in how they interact with each other, they parallel. Duncan gets the upper hand usually, with Harold occassionally dunking on him and turning the tables. They both think the other is a total idiot, but they're willing to put aside differences for the sake of the team. Some of Duncan's confessionals even imply that he believes that his bullying of Harold will make him actually toughen up, which makes sense when you factor in how Duncan is in-and-out of juvie constantly. It's probably how he ended up being so untrusting of everyone and sees enemies everywhere.
It sucks how these comparisons are lost on a lot of viewers, who boil their dynamic down to such a finite level that discredits all the work put into both characters. It's always 'Duncan is a loser you're meant to laugh at, because he's so pathetic!' and 'Harold is insufferable to deal with and rigged the votes against an innocent party, he sucks!' with Duncan definitely having more detractors due to people seeing themselves in Harold. Ironically, haters of either usually hate traits that both characters have. Harold isn't some innocent guy who's getting bullied for no reason, and Duncan isn't some one-note bully who mocks Harold out of jealousy. Their relationship has nuances and comparisons that mean a lot more than meets the eye.
