A good way to handle this would be that if both sides agree that they should be shooting each other IRP, its ok to let them shoot each other. If only one side thinks so and the other doesn't, it's not ok.
If one sided shooting of the same ID is permitted, it will probably mostly happen in ways that used to be a common sight when we still had players:
One side decided that they get to tell the other side what to do (ranging from "don't attack your IRP enemy who is my OORP friend" to "don't go to X or do Y because I R boss of it" to "dont fly caps, caps are only for me and my friends") because they are "stronger" (I'm a pro with friends in the right places and ur a noob). It's pretty much translates into "submit to me or shut up or leave", which is the attitude that is and was the root of most of disco's problems.