I've copied 'dbldash.patch' over from http://bugs.python.org/issue13922.  I mistakenly added it to a closed issue.

The patch is old, and should be revised.  It is also missing tests for this '--opt=--' case, even though it solves it.   

I also reference this patch in http://bugs.python.org/issue9571, which tries to refine dbldash handling for PARSER, REMAINDER nargs.

I'm raising the priority on this because the issue has a come up a couple of times on Stackoverflow:

http://stackoverflow.com/questions/40685320/python-argparse-with-as-the-value