There really is no perfect way to do that, because someone who wants to register another account can find a way to do it from a different ip address and there's no way to tell. Also, there are reasons why two legitimate accounts might have the same ip address (like two people who live in the same house). That said, there's this mod that checks for duplicate ips:
https://vborg.vbsupport.ru/showthread.php?t=282525