I am trying to figure out how to configure a form so that users only have to fill it out once...

What I have is a download page, before the user can download any files off the page they need to submit a form mail to me. The catch is I only want the user to have to fill out that form once.

Does anyone know of a method to do this? (cookies?)

If you are confused here is a sort of menu/tree that show's what I am trying to accomplish

first time visitor
download link > form page > download page

visitor who has registered
download link > download page

Thanks!!