title: Best way to avoid fake users
body: <p>I'm developing a web app where it's crucial that the users can't register twice (e.g., with different emails).</p>
  
  <p>I'd like to use some automated way to check it's unique.</p>
  
  <p>Checking Personal Identification Number (like in Passport), isn't a good idea, because it must be done manually, since gov sites use captchas.</p>
  
  <p>What is the best way to achieve it?</p>
  
  <p>To be more specific, what is a field that I can use with UNIQUE constraint in my users table?</p>