If you mean that you can't get damnation to hit con twice, I have no problem with that. If you mean that you can't damn someone more than once, as seems more likely, I have a problem with that. Shamans are not by any means an overpowered class. They do alright, but have very few unique features. One of the few unique things they do have is damnation. Since they can't choose what damnation will affect, it's alot weaker if they can now only do it once. If they get a -mvs damnation or something of the like, that's it, and all they really got was a harder to remove curse, with less useful effect. Being stackable, it had the advantage that a shaman could keep damning, trying to get useful effects.

It seems to me you took a decent chunk out of one of the few unique abilities of a shaman, in order to prevent a small situation that could only happen with the aid of someone else. There are many extremely powerful combinations of things that two players of complimentary classes can do. They shouldn't be the main considerations for whether to take power from one class as a whole.