It's about time...

[Coatl]

But you actually already can't use raise on someone who is already raised. They received a "raised" buff and it will cancel your raise if that buff appears.

It just won't stop two healers from swiftcast raising the same person.

[Tsukino]

Not sure this would fix too much. In many cases you're using Swiftcast before Raise/Resurrect, so even if it doesn't allow you to cast it you've still wasted your Swiftcast cooldown unless someone else is dead. And if you didn't use Swiftcast, you can tell they've been raised by someone else long before you finish casting.