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make the first account they log in to on that ip the primary account for the ip for a couple of hours and then make them answer extremely guessable security questions to log in to a secondary account
or you could keep the last account logged into and the time+date in a cookie and then check that
If it's free, you cant really do much to prevent people from operating two different accounts...
If I want to, I'll use a different ip address, different username(duh)/password(duh) and different email, different browser, etc...
The only way you could prevent it would be to charge... and even then if i wanted to I'd just use a different credit card (though you probably wouldnt complain )