having the same IP or using a VPN should not be the only reason a casino uses to accuse a user of multi accounting. i'm sure they look at other data, like where deposits/withdrawals are coming from and going, device fingerprint, etc...
if an accusation is merely based on having the same IP, it's pretty weak and unfair.
If you have been reading the terms of service of gambling sites, you will noticed that they always state that the same IP should be for one household and not just a gambler. That is why it is not good for members of the same family or people leaving together to use the same router to be gambling on the same gambling sites.