Many cases did found not just with 1win but also in other casinos that people claimed they have never made multiple accounts which is against the casino rules. The bonuses that they think are easy money, investigations are done with this and it's a process that they have to verify to make sure if the accused has abused the system or not. So, if there is the verdict that it has been abused and taken advantage of, they will surely say that it's done and deposits are likely to be refunded to the user but won't be allowed to play anymore.
Well this type of behavior can be stopped because the casinos can simply require captchas when registering and logging in. And not to mention the fact that VPNs are banned at most casinos for precisely this reason, and ID verification being required for withdrawal.