You can use the IP address of the submitter and store that as the unique identifier... but it is possible for them to go somewhere else and submit the form again... although this is the simplest way that I see.

I can have IP and emailId but what if he tries to use a different computer and different emailid, then again it will not work.

Well there is not much you can really do about that other then getting a phone number.... phoning them and confirming that it is that one person.

You can ask for credit card but then they have to have one to register and even worse you would have to store the credit card numbers which is something I do not recommend EVER.

I think with IP and email you have made it significantly difficult to have more then one account. Some advance users will be able to get around this but I would say 98% of your audience will be just stick to the one account due to your constraints.

What I would add to what you already have is put some text in there that explains that the name they register with must present ID with the same name to claim the prize.

You can do some manual searches of the database to see how many duplicate names you get. If there are a bunch of one that would raise a flag for me and maybe investigate further.

If someone wins but can't produce the ID of same name then they most likely registered under other names and so you can redraw for a new winner since they couldn't produce the ID.

Thanks that is a helpful suggestion