The problem was he shouldn't have spent so much time trying to find a counter, and instead used his most comfortable point character (I'd say Billy) and just use him throughout. He would've picked up on the matchup eventually and would've been good. But he spent so much time trying to find a counter, Duo was just allowed to run train.
People forget, Mad_KoF ALSO has a huge stable of characters he plays, but he stuck it out with one team, even when Bala got a 2-1 lead on him in the first set. Counter-picking will only get you so far. While you should have at least a 6 character stable, you should have 3 go to characters in ALL situations. Your true mains. Those are the ones you go to when the chips are down because you know how to play them, it becomes instinctual. When using characters that aren't your mains, you can be proficient, but compared to true masters of the characters your timing will be a millisecond off, your decision making slightly less clutch. With your mains, it comes natural. There's no thinking about the next step in matchups, you just do the step. That comes over playing them over and over again in all situations, something you can't do if you're constantly switching around characters.