text: Title: Extra in report is duplicated
  Body: ```
  import pytest
  @pytest.mark.hookwrapper
  def pytest_runtest_makereport(item, call):
      pytest_html = item.config.pluginmanager.getplugin('html')
      outcome = yield
      report = outcome.get_result()
      extra = getattr(report, 'extra', [])
      extra.append(pytest_html.extras.html('<p>Print something</p>'))
      report.extra = extra
  ```
  
  In the report, the text is displayed twice:
  ```
  Print something
  Print something
  ```