I understand that Marketo must use the email address as the unique identifier for leads but is it possible to use two unique IDs at the same time? We have multiple contacts that use the same email address but should have two records in our database; however, when we import lists, these records are automatically deduped and only the first record on the list import is actually saved to our database. We have an alternate field that serves as a unique ID for all of our contacts but I don't know how to set it as a unique identifier in conjunction with the email address so that there can be multiple leads in Marketo with the same email when we import.
You can request via a support ticket to use a secondary dup key for your id field so that Marketo will dedup based on email address + another field. You can also have this work only for list imports.
However there are some caveats and you need to be careful if you use this approach. With two dup fields, if someone imports a list with an empty value in the secondary field a new record will be created which could leave you with more duplicates than intended.