IP address is the most solid proof for multiple account connections since we are on the internet which means there’s no other way to identify our identification. KYC should be one of the solution but we know that it can be faked through services that offer KYC verification.
When I was at university, in the laboratories when we connected to the Internet they used a proxy, so many confused it with a VPN and it is not, there are VPNs that are powerful and able to fool any server, even so what they cannot fool is a public IP, but when we put airplane mode and with data on our phone its IP address changes, but what does not change is that you can see the physical address, something that is difficult for them to change.