It’s not possible for there to be duplicate logins or emails across multiple user accounts. What do you mean by “duplicate member”? The same person using more than one user account? We call additional accounts from one person “sock puppets”. The IP is a good clue much of the time, but it’s possible for different users in the same organization to have identical IP addresses, so it’s not a definitive clue by itself. AFAIK, there is nothing useful in cookies that would tell you if two accounts are for the same person.
It’s possible to collect enough disparate browser and computer data where it could be compiled into a sort of unique “fingerprint”, but it’s still not definitive. Two different coworkers could be registering from the same computer, possibly even the same machine login.
