title: python: argparse throwing value error when combining positional and optional argument
body: <p>I'm trying to use the argparse library in python to read in optional and required arguments. So far I'm doing this:</p>
  <pre><code>import argparse
  parser = argparse.ArgumentParser(description='Cleanup Script for Folder')
  
  parser.add_argument('PATH_TO_WORKDIR_ROOT',
                      type=str,
                      dest='PATH_TO_WORKDIR_ROOT',
                      action='store',
                      help='(absolute or) relative path to directory \n   to work on, e.g. &quot;..\MyFolder\\&quot;')
  parser.add_argument('--PATH_TO_BACKUP_ROOT',
                      type=str,
                      dest='PATH_TO_BACKUP_ROOT',
                      action='store',
                      help='(absolute or) relative path to Backup-Directory \n   default is &quot;.\BACKUP\&quot;')
  
  args = parser.parse_args()
  </code></pre>
  <p>Now I'm testing my code and it gives me a value-error that I don't understand:</p>
  <pre><code>$ python argparsetest.py --help
  Traceback (most recent call last):  
  File &quot;argparsetest.py&quot;, line 5, in &lt;module&gt;
      parser.add_argument('PATH_TO_WORKDIR_ROOT', type=str, dest='PATH_TO_WORKDIR_ ROOT', action='store', help='(absolute or)
  relative path to directory \n   to wo rk on, e.g. &quot;..\MyFolder\\&quot;')  
  File &quot;C:\Program
  Files\Enthought\Canopy\App\appdata\canopy-1.3.0.1715.win-x86_
  64\lib\argparse.py&quot;, line 1262, in add_argument
      raise ValueError('dest supplied twice for positional argument') ValueError: dest supplied twice for positional argument
  </code></pre>
  <p>There's only one positional argument, isn't there and the destinations are different. I don't really understand the hassle :)</p>
  <p>Thanks a lot in advance!</p>