title: Can't open "source.extension.vsixmanifest" file in Designer Mode
body: <p>I have Visual Studio 2012 Package Project (from VS2012 SDK). I can't remember what I have done before, but at this moment I get a following error when I try to open <code>source.extension.vsixmanifest</code> file in designer mode: "<strong>An item with the same key has already been added</strong>". I am able to open this file only in "View Code" mode.</p>
  
  <p>Moreover, what is weird, If I create another VS2012 Package Project in this Solution, the same error appears for the corresponding <code>source.extension.vsixmanifest</code> in the new project.</p>
  
  <p>It looks like a dictionary or resources duplicated key problem. But I haven't modified any resources or dictionaries.</p>
  
  <p>What is the reason of this error. How can I fix it?</p>