The problem with that sort of pre-fight tit-for-tat is that many times it's never really clear who did what first. Even when you've got witnesses, there's a different story from everyone, and it just isn't clear who the true instigator is. I think the best you can do is treat it as a regular interaction. People who were clearly defending themselves get a pass, but if it's one guy who starts it, but the other guy retaliates, they both get their wrists slapped. I think it's the only fair way to handle it.