Bz29146/30250 changed the behavior of the default values to make it so they are only applied when creating a new record so if someone wanted to delete a field that had default valued in the framework, those informations wouldn't reappear when the record was saved. 

The old behavior seems to still apply when Koha detects a duplicate record.

Test plan :

1. Have a framework with a default value in any non mandatory field. Make sure this field is visible in the editor. For my exemple I had "rdacontent" as the default value in subfield $2 of field 336.
2. Create a new record and delete the $2 that appears with my default value
3. Fill all the mandatory fields and in 245 write the title of an existing record in your catalogue to create a duplicate. 
4. Save the record. The page will refresh and you'll have the "Duplicate record suspected" alert asking you if this is a new record or a duplicate. 
5. Go to the field you deleted (336 in my exemple), the default value came back.

To compare, do all of the same steps, but create a record that has no duplicate in the catalogue. When you save the record, the deleted field is not populated with the default values, it stays deleted as it should.