I agree to most of this analysis. Except if you take it before you have a 20 in your primary.Most feats are overpowered relative to the alternative, because they're balanced against a +2 in your primary stat, which means any feat that you can use is flat-out better than +2 in any other (non-primary) stat. Once you've capped your primary stat, any feat you can use is a pure power boost. This feat just happens to be one which can benefit anyone.
In the base game, if your fighter wants to increase its Wisdom save, then you take Wisdom +2 at your next opportunity. Once feats are available, your fighter just takes this feat instead, for roughly five times the benefit. If you can take the feat twice, then you're even more overpowered than you would otherwise be.
So yes, it would be broken to let someone take this feat twice, but it's already broken to let anyone take it once. It's just a question of degree, on how broken you want your game to be.
Not to mention how Paladins eventually get a +5 on all their saves...The monk eventually get proficiency in all saves as part of his progression.
This boils down to the question "how minmaxing are your players?" Yardiff.I allow it to be taken multiple times. It doesn't really break anything. And I don't see a player taking it more than twice unless someone is trying to build something really unorthodox like a grappling wizard. If someone wanted to play that at my table, why not give the player all the tools to be able to build it?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.