Well, that's why I said the concept would have to be implemented with special rules:

a) Double Cast simply wouldn't work on Enhancing and Enfeebling magic, or ...

b) The Trait/Ability is programmed such that it couldn't proc on a spell that has successfully landed it's effect (such as a successful Paralyze or any Enhancing magic), or if the target's HP has reached 100% (in the case of Cures).

I didn't suggest any change in mana costs since, essentially, you are casting the spell twice, just without the manual effort or recast timer. Of course, with that kind of penalty, it might be better off as a Job Ability rather than a Trait.