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I'm building a site that needs to guarantee user reputation scores are accurate by preventing users from creating more than one account, at the cost of decreased user signups. So far, the only solutions I have thought of are allowing users to:

  • Link to their (verified) PayPal account through PayPal Account Authentication
  • Provide their PGP public key, and checking that the MSD (a metric of trustworthiness) is below a certain value

Of course, even these methods aren't bulletproof, but are likely to make creating a sock-puppet account very difficult. Are there any others I haven't considered?

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3 Answers 3

Charge them real money for each account.

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True, even if they do create a second account the site owner gets paid, twice. This won't work in every situation but it's a great way to cut down on spammers and trolls if you can do it. –  jfrankcarr Oct 19 '11 at 18:48
    
You could also record a hash of their cc # and cc name and billing zipcode and check for duplicates (of even just the cc name and billing address) –  Clay Nichols Jun 12 at 21:51
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From this StackOverflow question:

  • Place cookies on the users computer
  • Log their IP
  • Store content into their localStorage (only for users with HTML5 browsers)
  • If you really want to start digging deeper, you can start putting restrictions based on the users session length, how many pages they navigated prior to voting, i.e. starting to profile the users that try to circumvent the system, and start putting restrictions on those profiled users.
  • MEMORY tables on server with ip addresses
  • evercookie
  • browser fingerprinting
  • required registration
  • http://code.google.com/p/mailvalidator/
  • make list of banned domains
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How about two legitimate different users login from same computer? –  clinisbut Oct 20 '11 at 19:48
    
Evercookie looks useful for this. –  Clay Nichols Jun 12 at 21:52
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I think there are varying degrees of bulletproof. So other ones might include: - Linking to a Facebook account - Google account - other Oauth providers

It's a tradeoff between making it so hard to get an ordinary account when you aren't trying to game the system vs the ease of those who want to creating duplicate Facebook profiles/PayPal accounts.

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