Double user

We came across this problem recently... A couple of our users registered again as different users to respond to a post without the fear of banning. But the cunning admin found them out. Is there any way I can check if a user with the specific IP has ever logged with another account with the same IP at that time... And I imagine it as an Administration Panel thing so I can check all the users at once.

Thank you in advance

If the users posted you can click on the IP of the post and show you the IP of it. Also you can ban the IP and they shouldnt come back.