title: ConfigParser get all keys with duplicates
body: <p><code>ini</code> file:</p>
  
  <pre><code>[main]
  key_1=1
  key_2=2
  key_1=3
  </code></pre>
  
  <p>Python:</p>
  
  <pre><code>config_parser = ConfigParser()
  config_parser.optionxform = str
  config_parser.read('config.ini')
  for section in config_parser.sections():
      for key in dict(config_parser.items(section)):
          print key
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>key_1
  key_2
  </code></pre>
  
  <p>Expected result:</p>
  
  <pre><code>key_1
  key_2
  key_1
  </code></pre>
  
  <p>How to achieve such a result?</p>