However, there are a few exceptions:
* Rival: You can battle your rival multiple times throughout the game, but these battles are part of the story progression and not optional.
* Gym Leaders: After you beat the Elite Four, you can rematch all Gym Leaders at their respective gyms.
* Elite Four and Champion: After beating the Elite Four and Champion, you can rematch them at the Victory Road entrance.
So, while you can't battle the same random trainers twice, you can rematch the main story's important figures after you've finished the game.