When External identifier is imported and duplicates are set to 'skip', CiviCRM reports dupes as errors, not duplicates

I am importing from an external DB via CSV and have external identifier set. I Use the same dump for weekly imports, which will have some extra people in it occasionally. This file needs to be imported in stages, with stage 1 being:

• Skip updates
• match on a dedupe rule that uses external identifier

I would expect duplicates to be flagged as such, but instead, they cause an error for each line at step 4: Invalid data and/or formatting which only says that there is a duplicate External Identifier in the CSV download of errors.

I would expect that the code block at line 480ish of /CRM/Import/Parser/Contact.php would return CRM_Import_Parser::DUPLICATE