We are going through a database cleanup right now where we are merging duplicate leads. Our whole database isn't synced to SFDC, so first we are merging duplicates in SFDC, then we are merging duplicates in Marketo. Unfortunately, not all of our Marketo fields are synced to SFDC either which poses a bit of a problem when choosing with field to keep vs. which field to overwrite. I know that Marketo handles this for some fields (see article here and ideas and ) but there's no way of determining behavior for other fields.
My idea is that we should be able to set master merge preference for specific (or just custom?) fields within field management. Merging leads is already a very manual process unless you're willing to pay extra money for Marketo's Easy Merge service, and the merge leads window to help you choose which data you should maintain on which record. These preferences would be maintained as standard merge preferences, similar to the way Marketo adds values for lead score fields.
For each field you could choose from things like:
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