If your are only updating one field, you could also deal with this using smart campaigns, I am not sure how many people you are dealing with.
Copy in the email addresses or the domains of the emails ( I am not sure if the product interest varies per company or per individual user) and change in the flow change the data value of the product interest.
Marketo should de-dup but it's always good to have a workaround when you run into these types of issues!
Hope that helps!
-Ryan
After reading through all of the responses, the only other thing I would suggest would be to make sure that when you are importing the list of records you are associating the 'email address' field in your spreadsheet with the CORRECT 'email address' field in Marketo. For a long time I would associate 'Email Address' to 'E-Mail' and end up with duplicate records - so make sure the correct field is being synced!