How to capture multiple leads from a third party using a single email address

Hi,

We have identified an opportunity to grow our business through third party referrers however have come into an issue with capturing leads.

We are finding that when our third part referrers are sending us leads they will go onto our site and fill out a new form using their own email address. They are hesitant to provide the leads details until further into the journey to ensure they are compensated. Using their own email address causes issues and new leads being sent through are overwriting the earlier leads. 

I'm wanting to see if there are suggestions on changes we can make withing marketo to salesforce to be able to capture all the leads.