Hey Karly,
I just asked tech support about this last week. I had a few instances where a contact in SFDC registered for a webinar using different email addresses and they were created as new leads.
After doing some research, it sounds like there isn't a great way for Marketo to de-dupe in this case -- at least that's what I heard. I did, however, use it as an opportunity to build a little alert system for myself using this tutorial from the community. It's not perfect, but at least helpful.