There's difference between intentionally and accidentally multi accounting creation, Since if situation happens that the user maybe forgot if they have an account of that casino especially if the casino exist for many years and they are not playing on it for long time.
If they just create in good faith like they just want to play and no sign of abuse happened, maybe the casino will fix the issue and allow them to gamble. But if it happens that they create another account to gain advantage on running promotion and abuse their bonuses in that case they might get severe penalty from the casino.
Accidentally do multi accounting can happen but it's barely chance and people are always able to proactively contact support for their concern of possible previous account they had on the casino but forgot its registration and login information. Perhaps when people proactively ask for support, and announce their concern, they will be likely whitelisted and won't have issue with their newly created account. If anything happen, the casino will be more friendly as they were informed by the user on possible long time abandoned account which should not affect the new account of that person.
Surely it's best to recover login information and use the previous account if the person can do that account recovery.