There are so many way to catch a multiple accouint, it will startrs with IP and the gadget ID they are using.
The bank accounts used in depositing money to play the casino, but, what if? what if they are brothers or relative who load in one account coz the other dont have but want to play?
living in the same house? will that be counted ?
The only focus must be that if player gain any edge over the casino by using multiple accounts. Though every site has this anti-IP collusion clause in their terms fo safe side but IMHO no reputable casino will deny your fair winnings just because of multiaccounting.
Actually many big sites don't pay winnings when the user has multiple accounts. Often enough the best case scenario would be to get the deposit back, that's about it.
Of course this often is quite unfair unless the player actually had any advantage through having several account. But are there any? Sure, if you got limited on sports and create a new account to circumvent those limits that's not a smart move. Also if you had any first deposit bonus or whatever and used this for multiple accounts that would also be a solid ground to suspend an account.
But other than that I don't see any reason why players shouldn't be able to have more than 1 account. Playing the casino has literally no advantage if you use 1 or 2 or 25 accounts, there is no difference. Casinos should be happy for every deposit they get, doesn't matter if it's from the same person but a different account.