title: Why does Microsoft Word fire the DocumentOpen event for an already opened document
body: <p>I have a Microsoft Word Add-in using VSTO and interop. My code is having issues if the <code>DocumentOpen</code> event is fired for an already open document. I've been able to consistently replicate this by opening a document doc1, opening another document doc2, and then trying to open doc1 again. </p>
  
  <p>Doc1 is already obviously open, but the DocumentOpen event is fired again for doc1. Is this an intended design and if so, is there a simple way I can tell if the event is being fired for an already opened document?</p>
  
  <pre><code>private void ThisAddIn_Startup(object sender, System.EventArgs a)
  {
    try
    {
      this.Application.DocumentOpen += 
        new MSWord.ApplicationEvents4_DocumentOpenEventHandler(Application_DocumentOpen);
  
    }
  }
  
  private void Application_DocumentOpen(MSWord.Document document)
  {
    HandleOpenedDocument(document);
  }
  </code></pre>