We have 2 main sources of lead creation - leads coming from Salesforce, and leads being uploaded to Marketo via List Import.
Sometimes we are finding that duplicates are being created when firstname.lastname@example.org was originally created in Salesforce, and synced over, as well as when it was imported via list. My understanding is that if the way an email address enters into Marketo in 2 different ways (Salesforce vs. List Import for example), Marketo will not keep the email and overwrite everything else.
Is there any way to prevent this? Can I make sure that when an existing email address, which came from Salesforce, is later added again through List Import, that a duplicate record isn't created?
Marketo only dedupes when the email is entered directly into it from Lists, API, or Form Fills.
If SFDC Lead is created and passed to Marketo, no deduping occurs.
But if that Lead is already synced to Marketo, and the matching email ALSO appears on your list import, it will dedupe.
So I suspect that your records in SFDC are somehow created after your list imports. Or that you have some other logic going on that's creating these.