Collecting unique emails

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?

Most email marketing services will have this option enabled on the forms you can create within their dashboards.. sendinblue, mailchimp etc all do.

I used Lead Grabber to handle this.

Most list applications/websites will automatically eliminate a duplicate email address as being an additional lead. Beyond that there are scrub procedures to clean up invalid lists. There are tons of ways to clean your list if you continue to have this kind of problem. By your description it seems you may be inaccurately asking your question as most people here will think you are talking about an email list, when it seems more likely you are talking about a PTC campaign or an actual random drawing script which was possibly extremely poorly scripted.

More detail may make it easier to shed light on your exact problem/issue.