So, there are a couple options. One, is maybe you can alter the process in SFDC, but if that isn't going to fly you can set up Marketo to make similar duplicates- deduplicating on email address and another field like "product interest"
In general, this stuff is somewhat hard to get set up the right way, so I'd really recommend help from a consultant, either someone at Marketo or someone who is very familiar with the system.