Prevent duplicate lead creation from web to lead

We are using web to lead creation and it seems to work well. A problem we have is when a prospect fills out 2 or more forms on our website and clicks submit. This will create a duplicate or even triplicate lead with the same email address and user information. Is there a rule we can run that will prevent a lead from being created if a lead with the same email address already exists? We would like it to create a new lead if the previous lead is 30 days old or older but not if the first lead was created inside that time frame.

Thanks,

Jeff