Marketo dedupes by email address. So if you have a bunch of leads assigned to none@ourdomain.com, they will be considered matching dupes regardless of IP or Name. This assumes the leads come into Marketo FIRST.
So if you could have a situation where you create leads in SFDC with that email, those get synced to marketo. Then someone is uploaded to Marketo with that email, gets deduped against one of the emails and syncs back. Usually Marketo does this with the most recent leads.
I would caution against syncing leads from marketo to SFDC w/o an email (which you realize), but just not even pass them over because of this risk. Or maybe have a trigger in SFDC that adds a unique # to the lead's email.