Any idea about how to do this? [contact forms related]

Hi, I am going mad while looking a solution to this but I don’t really know which plugin I could use.

I need a contact form where people have to subscribe to our newsletter in order to claim a coupon for a free lunch in a restaurant. Only the first 100 people will have the coupon via a bounceback email that has to be sent automatically once they submit their email. All the other people from the 101 on, will receive a different bounceback instead.

Also we need to prevent multiple submissions.

Does anybody have a clue?

Thank you very much!