I use Prosper 202. It is not 100% accurate but it has a nice interface that shows you duplicate IP's. It is a lot better than looking at server logs. It also does a bunch of other stuff for affiliate marketing and such.
Thanks for both the question and the answer - I'm going to check Prosper 202 out as well. Thanks again.
ya. great post... I get a little irritated paying 1-3 bucks a click and wondering if I'm getting scammed by google.