https://bugs.python.org/file29845/dbldash.patch

while written against an earlier version of `argparse`, does what you want.  I moved the '--' removal out of `_get_values` and into `consume_positionals`.

Note that a full patch should include test cases and documentation changes if any.  

Also now github pull requests are the preferred patching route.  But don't expect rapid response on this.  The issue has been around for a long time without causing too many complaints.  

Backward compatibility is always a concern when making changes to core functionality.  We don't want to mess with someone's working code. 
 Though I doubt if there are many users who count on multiple '--' removals, one way or other.  The argparse docs explicity calls this a 'pseudo-argument'.