Hello Emil,
You should use the defOnDuplicate="true" or "false" in the schema which holds the entity .
The defOnDuplicate function if set to true , uses the default value of the schema instead of the stored value in the record being duplicated.
Examples can be found in the nms:delivery OOB schema like :
<attribute defOnDuplicate="true" desc="Start date if action is deferred or delayed, validity date otherwise."
label="Contact date" name="contactDate" type="datetime"/>
Regards,
Adhiyan