Duplicates the specified example.

    Args:
      request: A request that should contain 'index'.

    Returns:
      An empty response.

def _duplicate_example(self, request):
    """Duplicates the specified example.

    Args:
      request: A request that should contain 'index'.

    Returns:
      An empty response.
    """
    index = int(request.args.get('index'))
    if index >= len(self.examples):
      return http_util.Respond(request, {'error': 'invalid index provided'},
                               'application/json', code=400)
    new_example = self.example_class()
    new_example.CopyFrom(self.examples[index])
    self.examples.append(new_example)
    self.updated_example_indices.add(len(self.examples) - 1)
    self.generate_sprite([ex.SerializeToString() for ex in self.examples])
    return http_util.Respond(request, {}, 'application/json')