and the casinos are not always providing proof of the other account when they make the accusation.
The important thing here is whether if the accused can prove himself to be innocent or not. He’s gonna need to present evidence and report claims to the casinos which I am certain that if this casino was legitimate they would not simply ignore your claims and concerns.
If you need to submit personal information to verify you only have that one account then be prepared to do so. You never know maybe the casino is telling the truth and there really is someone else that registered under your name.
Interestingly enough, there is something called dynamic IP addresses, which are rather common in some countries and also depends on one's Internet provider. In the case of my country, most of IP addresses are dynamic, that means we never have the same IP address when we access and browse on the internet. So in theory, there is a small but non-zero chance someone using a casino account in the same website that I am, ends up connecting through the same IP address I used, which would mean we both could be accused of being the same person using two accounts to gamble. In those cases it would be very difficult to prove one's innocence, specially if the staff of the casino is not familiar with the state of IP addresses in one's country.