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.
They will talk about a reason of don't know about that rule against multi-accounting on a gambling site. This is a most common reason used by people with multi accounting activities for making their ToS violation like naive and unintentional. I agree that there are people don't know about that rule but they are newbies and not too many gamblers like that. Most gamblers are experienced and they know what they are doing is ToS violation but they believe that they are experienced enough to outsmart and break ToS of a business without consequence.
When their activities are detected and their accounts get consequences, they complain and try to make them like naive users with lack of understanding on ToS and multi-accounting rule.