Show message when 2 or more commenters have same IP

Maybe this is already a plug-in but I could not find it.

I want to display something near the comment when comments are posted on my blog on the same post and 2 or more commenters have same IP. Like user1 has the same IP address as user2 and user3.

Why?

On a political blog I run a few users are using different user names but are the same person and trying bolster their viewpoints by agreeing with themselves. I don’t want to disallow same IP address comments as some are going to be legitimate. I just want others reading the comments to know so they can take that into consideration.

Any one have any ideas? I would think this is a common problem and already solved in some way.

Thanks