I have been interested in email marketing for quite some time now. But I have a question that I have never seen answered before: how can you collect emails that aren't already in your database? For example: I have a giveaway - if you enter your name and email you have a shot at winning GTA 5. But you really really want that game so you just spam the form with the same info over and over again which greatly influences your chances of winning. How can I stop someone from entering the same email twice?