I am developing a survey site where everyone can vote once. Obviously, I have to prevent multiple registrations for research in order to stay relevant. I force each user to log in with their Google, Facebook or Twitter account. But they can be authenticated 3 times if they have an account on each or authenticate with multiple accounts on the same platform (I have 3 Google accounts). So I decided to keep my IP address, but they can still use a proxy server. I could support the HTTP HTTP agent with PHP get_browser () , although they can still change browsers. I can extract the OS with regex to make changing the OS easier than browsers. And there is also geolocation, for exampleMaps API the Google .
You can check the user by mobile phone by sending him a text message with some code. This will limit the vote count to count the number of mobile phones owned by the user.
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Requiring users to telesignin for an account seems like a fairly common way to prevent bulk registrations. There are several companies that provide these services.