Security- Preventing duplicate people

[hc1619]

I am building a referral system for my site. Members will be able to refer friends to my site (via a mechanism I'm building), and if 5 join up, then I'll give that member 1 free week of paid membership.

I want to prevent people scamming the system... stopping people from just signing up repetitively on their own computer with different accounts. I know I can't completely prevent this, but what are some good measures I can take? Other than checking for duplicate email addresses (which I already do).. what else can i do? Logging their IP and storing it is useless isn't as the IP changes every time. What can i do?

 

[loydwilson]

I utilize a series of cookies placed on user machine that application looks for each time a user attempts access. I utilize a control code containing a determined algorithm to help control usage.

 

[TruthInSatire]

You could wait x months, weeks or days after the 5th account is created. Check how many DB transactions were made by the 5 "users" if there isn't any/many you can assume they are a fake user and deny the bonus

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