Here is additional information about native de-dupe behavior for form submissions to supplement the great article John Mattos wrote about "The Unintended Consequences of Duplicates in Marketo."
During form submissions if the record isn’t cookied and their email address is in Marketo multiple times, Marketo will match with the one which was most recently updated (by last modified date in the “updated” field).
There is one exception to the rule highlighted above. If the unsubscribed (SFDC opt-out) field changes to true, Marketo will automatically unsubscribe all records with a matching email address. Thus, if the SFDC opt-out field changes to true all records with the matching email address will change to true. The data value change will be noted in the Marketo activity log for each of the records.