It's fair enough to say that a casino shouldn't allow it in the first place and the accounts should be blocked immediately when the gambler creates two or multiple accounts, but why does a gambler have multiple accounts in the first place? If you have multiple accounts, and you hit a big win with one of them, and then the casino investigates and finds out what you have been doing, they have all the rights to ban you or confiscate the funds.
I do agree. It may be written in TOS or just be seen as a fraud and treated as such. Nothing personal for the one who did such things 
After all, casino will know if their members cheat them because they have all record about their members activity so they can check it to get more information about their members. Casino can investigate easily and casino will asking their members to verify their account by sending the other document to make sure they have a real document.