Web form to Lead creates duplicates if contact already exist

Web form to Lead creates duplicates if contact already exist

We get leads via our web form, and sometimes they already exist as a contact. In this case, creates a new record as a lead resulting in duplicate data.  

It also sends the lead to be approved so it's hidden and has caused us to not respond in a timely manner if we don't see the email notification.  

How do I effectively get incoming leads merged with existing contacts?

Thanks!