I am working on a website that requires a bit of marketing; let me explain.

This website is offering a single, say, iTunes 50$ voucher to a lucky winner.

To be entered in the draw, you need to invite (and has to join) at least one friend to the website. Pretty straightforward.

Now, of course it would be easy for anyone to just create a fake account and invite that account so, I was thinking of some other way to somehow find out of possible cheating.

I was thinking of an IP check on the newly subscribed (invited) user, and if there is the same IP logged in the last 24 hours, and if that's the case, investigate more about it.

But I was thinking that maybe there is a more clever way around this issue. Has anyone ever though about this? What other solutions did you try?

Thanks in advance.