title: PyPDF2 nested bookmarks with same name not working
body: <p>When you try and nest several bookmarks with the same name, PyPDF2 does not take it into account.
  Below self-contained python code to test what I mean (you need at have 3 pdf files named a, b and c in the working folder to test it out)</p>
  <pre class="lang-py prettyprint-override"><code>from PyPDF2 import PdfFileReader, PdfFileMerger
  
  
  def main():
      merger = PdfFileMerger()
      first_one = True
      for file in [&quot;a.pdf&quot;, &quot;b.pdf&quot;, &quot;c.pdf&quot;]:
          print(&quot;next row&quot;)
          reader = PdfFileReader(file)
          merger.append(reader)
          if first_one:
              child = merger.addBookmark(title=&quot;blabla&quot;, pagenum=1)
              first_one = False
          else:
              child = merger.addBookmark(title=&quot;blabla&quot;, pagenum=1, parent=child)
  
      merger.write(&quot;test.pdf&quot;)
  
  
  if __name__ == &quot;__main__&quot;:
      main()
  </code></pre>
  <p>I would expect the resulting pdf to have three levels of nested bookmarks</p>
  <pre><code>blabla
      blabla
          blabla
  </code></pre>
  <p>but instead I get</p>
  <pre><code>blabla
      blabla
      blabla
  </code></pre>
  <p>Is there any way to make sure this does not happen ?</p>
  <p>EDIT : I have removed the <code>pagenum</code> variable as I want those 3 bookmarks to point to the same page.</p>