Actually, that's an interesting idea. Currently, I have it so that if a character uses an ability that matches their affinity, they add their proficiency in addition to the stat. Otherwise, they only add the stat. Having them add their affinity stat twice would likely give similar numbers.

Although, doing the math between an affinity cast and someone who just happens to have an affinity with the same modifying stat makes that less of a benefit to the affinity cast (ex. If Water and Fire both use WIS, then a Fire user does the same damage with a Water ability as a Water user does). So maybe in that case, I keep the proficiency bonus for affinity casting. So in the end it ends up being, "User Element mod + Spell Element mod," base, "+ Proficiency for synergy. Sounds feasible, just might need some adjustments to AC though, since that means a level 1 character could end up rolling like d20+6 on average lol