There are many ways to catch people out. People often try to change up the obvious ones like IP address and account details, there are more subtle ones that can trip people up like writing style or even computer settings (browser, resolution, etc). Ultimately they don't need to figure out much, it's only at the point of cashing out they need to be sure of the final recipient. Sure people can use friends but that introduces further risks to the end user trying to multi account, like theft or failure to produce documents.
Better doing a burners account or having multiple accounts its more ideal to read the terms and agreement with the platform because there is a lot of gambling platforms pretending the user to create a dummy account and become abusive to their platform, sometimes they freeze those detected accounts might cause preventing the user to make a withdrawal request. If they allowed with the use of make sure you manage properly and make the proper information to make sure the platform is aware on it.
^ Probably the gambling casino will take time to catch user that has multiple accounts and I think those who are already abused will probably easy for them but of course, they must be careful on this, it should have proof and not only by the doubt, it is very easy to use VPN and different device to log in your alternative account without knowing the platform, just like Stake.com that you can send a tip to each other user and who knows there someone there who send a tip to itself and that is the alt account. However, people who will be doing this abuse are those seeking money on gambling, not true gamblers.