Can't update or install package: An item with the same key has already been added : <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>
