This happens mostly with Saving Grace, but it actually happened with Chain Lightning too. Sometimes after you cast one of these spells, the animation and the effect happen, and then ~0,2 seconds later, the animation and effect happen again, as if you cast 2 of the same spell in a row. Happens about once per 25-30 games to me personally, not very frequent.
I suspect the bug could have something to do with the fact these spells have multiple effects and one of those effects is slightly delayed, and then maybe a glitch in network traffic could make it resolve awkwardly. Server doesn’t realize client already started resolving the spell (because it didn’t finish yet) so it sends the information to cast it again.
edit: I’m playing from an iPhone 6s + , with up to date iOS