We recently discovered an issue. When uploading a list of Contacts in SFDC with Data Import Wizard, or when Contacts are pulled into SFDC from ZoomInfo, Marketo then creates a duplicate Lead record.
How do we prevent this? I understand we likely should do all list uploads through Marketo, but how do we prevent this from happening when a Contact is pulled into SFDC from ZoomInfo?
As long as people can flow in from SFDC who already exist in Marketo, you can't avoid this happening. You have to merge people on one side or the other, potentially using an auto-merge tool.
So we are actually seeing this when a net-new person is added to SFDC (who doesnt exist in either SFDC or MKTO)
A single new record in SFDC, not created anywhere else (i.e. not simultaneously created in Marketo because of form misbehavior/misdesign) is creating 2 records in Marketo?
In a typical native sync setup, SFDC will sync your records to Marketo with the SFDC Id value as the unique key. If in Marketo a record already exists with the same email address, this is not recognized by the SFDC sync as the SFDC Id will be blank (record did not exist in SFDC yet). Blank is not the same as the value coming from SFDC and hence the match is not made. That is why the record will be created as a duplicate in Marketo.
If you were to upload them into Marketo with a list upload, normally the email address would be used as the unique key and the records would be matched. In that scenario, you would need to implement a small smart campaign to trigger off the add to list activity and sync the lead to SFDC if not already existing.
A first check to do is whether no alternative settings have been implemented in your instances related to the unique keys. If not, uploading through Marketo looks like the solution with the least manual work involved.