I'm not looking for something to check IP's against a list then ban them, I'm wanting it to check the who's online now and if 2 people have the same IP but different user names then put a temporary ban on them.
There should be no reason why 2 user's should have the same IP at the same time, also the chances of 2 people that visit the same forum that have the same ISP that live in the same area to get allocated that IP must be pretty slim
That's can possible... for example, one internet cafe usually have the same IP.