Yes, I know that too. Casinos don't allow users to use multiple accounts. In some cases, if a user is detected using different accounts but using the same IP address, they will suspend the account.
Casinos also have rules against this. I don't know the exact reason, but it's certainly to be avoided because it could lead to fraud or other undesirable incidents. I think if we have a clear reason, the casino will make it easier to close an account and create a new one.
This issues keep happening around that's why gamblers need to be aware that having multiple account is been discourage or not allowed to do.
There are lots of abuse involving multi account usage that's why the casino do some counter action to fight this. That's why people need to be aware on this so that they won't get compromised. Most of the casinos right now are so strict towards this that's why we see lots of people got compromise when they caught up having a multiple accounts.
If it happens that they have old account but can't remember the password the best solution with this is to contact their support since for sure they would help them to solve this issue and get clearance to gamble without getting any further issues.
If you just forgot your password, I think it's a simple matter. You just need to reset the password on the login screen, and they'll send you a verification email to reset your account password.
However, if you forget your email or login credentials, you can contact the service directly. However, I don't think that makes much sense, because you'll likely always remember the email you used to register. Furthermore, the email is tied to your phone, unless you lose all access to it, including your phone, due to loss or other reasons.