New CDO record in Eloqua removes previous record's mapping to the same contact

Hello,

I'm struggling with mapping multiple Custom Data Object records to a contact. I've probably missed a setting somewhere - could you help me troubleshoot?

My program canvas uses a form submit app to create new CDO records for contacts. One contact email address can have multiple records in the CDO, so the form used in the form submit app doesn't use any Key Field:

As expected, whenever the form submit app creates new CDO records, a new CDO record is created even if a record already existed with the same email address, and the new record is mapped to a contact.