title: try to avoid multiple instances of eclipse editor opening same file
body: <p>I registered a custom editor with eclipse extension point "org.eclipse.ui.editors" as follows.</p>
  
  <pre><code>&lt;extension
       point="org.eclipse.ui.editors"&gt;
       &lt;editor 
              class="com.xxx.designer.scxml.ui.ScxmlDiagramEditor" 
              id="com.xxx.designer.scxml.ui.ScxmlDiagramEditor"
              extensions="scdiagram"
              name="SCXML Editor"&gt;
  &lt;/editor&gt;
  </code></pre>
  
  <p></p>
  
  <p>It works fine, this editor is associated with .scdiagram files. However, every time I click on one same .scdiagram file, it opens a new instance for me instead of highlighting the opened file. Any idea on this?</p>
  
  <p>Thanks,</p>
  
  <p>Jie</p>