title: System.Collections.Concurrent already has a dependency defined for System.Runtime
body: <p>The error can be generalized as this when installing or upgrading a NuGet Package:</p>
  
  <blockquote>
    <p>...already has a dependency defined for...</p>
  </blockquote>
  
  <ul>
  <li>Using Xamarin Studio version 6.0.1, build 9 (although this error is not necessarily tied to any Xamarin Studio in particular)</li>
  <li>Attempting to load MvvmCross 4.2.1 (although this error may occur on any number of NuGet package)</li>
  </ul>
  
  <p><strong>Disclaimer:</strong> <em>There are a lot of possible answers on SO to similar questions, none of which address the root of this problem</em>. Questions are framed as a <strong>NuGet Package Manager</strong> version issue, with solutions geared toward updating said <strong>Package Manager</strong>, which is simply not an option with, say, the Mac version of Android Studio. </p>
  
  <p><strong>Question:</strong> Assuming I am running the latest, stable version of Xamarin Studio, and all I want to do is install a compatible NuGet, what does this error mean and how do I pick a compatible package?</p>