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Dont know the answer to your question so an alternative...
As part of the registration process have them enter a valid Email address. The email address is used to send them the password with which they log in for the first time. They can then change their password. Don't allow duplicate email addresses in the system but retain a log of them.
Ok some people will have multiple email accounts but then people could have multiple computers as well.
Only real downside which I know of is that people have to read their email to get into the system.
Combining this functionality with storing Ip addresses would mean less multis.