create a multirow assignment EIT using HDL

Can anyone share the sample file of creating assignment Extra info row (which is configured as multirow with unique key combination) for an employee using HDL load. We know after 23B Assignment EIT load has been updated with Workterm and Assignment lines in HDL. But still if there is any existing Assignment EIT but again we need to create another row with different combination of unique key - in UI it is allowing but in HDL we are getting error "You can't update this record because the SourceSystemId and SourceSystemOwner are invalid." Because it is not taking into consideration that