title: Can't update or install package: An item with the same key has already been added
body: <h3>Problem</h3>
  
  <p>In a particular project, I can't update or install any NuGet packages. When I try to do so using the NuGet GUI, it does some work and then stops without saying anything. When I try to do so using the package manager console, I get this output:</p>
  
  <pre><code>PM&gt; Update-Package –reinstall EntityFramework
  Attempting to gather dependencies information for multiple packages with respect to project 'SmartCentre', targeting '.NETFramework,Version=v4.5.2'
  Update-Package : An item with the same key has already been added.
  At line:1 char:15
  + Update-Package &lt;&lt;&lt;&lt;  –reinstall EntityFramework
      + CategoryInfo          : NotSpecified: (:) [Update-Package], Exception
      + FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PackageManagement.PowerShellCmdlets.UpdatePackageCommand
  </code></pre>
  
  <h3>Environment</h3>
  
  <ul>
  <li>Visual Studio Professional 2015 Update 1</li>
  <li>NuGet 3.3.0.167</li>
  </ul>
  
  <h3>What I've tried</h3>
  
  <ul>
  <li>Deleting the <code>packages</code> folder</li>
  <li>Restarting Visual Studio</li>
  <li>Restarting the computer</li>
  </ul>