I run a non-Bitcoin web site and use browser fingerprinting to detect the same user using different accounts. They can use VPNs, TOR, proxies, etc. but if they use the same computer/browser they will have the same fingerprint. No need to check IPs, which is useless in many cases anyway, e.g. for some companies if you log in from work 10,000 people share the same IP address.

Your browser fingerprint appears to be unique among the 5,095,764 tested so far.

Currently, we estimate that your browser has a fingerprint that conveys at least 22.28 bits of identifying information.