What is stopping people from registering more than once?
Hello! I'm still thinking about the best way to prevent this from happening.
First I thought that checking the IP address would be fine, but this would work only for people with static IP. Also, they could register more than once using a VPS, proxys, etc.
So now I'm thinking that every member should get a unique code which can be used to register. They would need to submit a google form with their Bitcontalk profile, and the account must be an active one registered before the 1st of September. Once I check this, they would get the code. This is more time consuming, but it will be fair for everyone.
If anyone has any suggestion please tell me!

You can't fully prevent duplicates. Stellar can't even prevent them with facebook authorization because people just buy facebook accounts. This will continue to be a problem until the resources gained through duplicate registrations exceed the effort required to receive them.
What prevents people from selling the unique code they receive? Basically, everyone will get ripped off if they're not gaming the system.