title: How to write an INI file with ConfigParser with duplicate options
body: <p>I want to write an INI file with duplicate options,ie:</p>
  
  <pre><code>[test]
  foo = value1
  foo = value2
  xxx = yyy
  </code></pre>
  
  <p>With <code>ConfigParser.set</code> only the last value is writed.</p>
  
  <pre><code>config = ConfigParser.ConfigParser()
  config.read('example.cfg')
  
  config.add_section('test')
  config.set('test', service['foo'], service['value1'])
  config.set('test', service['foo'], service['value2'])
  config.set('test', service['xxx'], service['yyy'])
  </code></pre>
  
  <p>The result is:</p>
  
  <pre><code>[test]
  foo = value2
  xxx = yyy
  </code></pre>
  
  <p>Is there any way?</p>