Looking at the test fixtures that exercise argparse, it appears that the intended behavior when '-' is not a prefix_char is to accept a doubling of any of the prefix_chars for long arguments.  That is, if '-' is not present in prefix_chars but ':' is, then an argument like '::help' would be a valid long argument.

I'm attaching a patch which fixes the originally reported problem as follows:

* If '-' is in prefix_chars, then the help and version arguments are '-h', '--help', '-v', '--version'
* If '-' is not in prefix_chars, then the first char in prefix_chars is used to lead the option, e.g., '+h', '++help', '+v', '++version' if prefix_chars starts with '+'.

Catherine Devlin is also sprinting here at PyOhio and will have a test fixture separately.