title: Duplicate interface definition for class 'NSValue'
body: <p>I'm getting a very strange error trying to compile my project, I get an error in <code>NSValue.h</code> saying 'Duplicate interface definition for class 'NSValue' (and NSNumber).</p>
  
  <p>I've tried cleaning and rebuilding, I've tried clearing the module cache using:</p>
  
  <pre><code>rm -rf /Users/me/Library/Developer/Xcode/DerivedData/ModuleCache/*
  </code></pre>
  
  <p>I've tried removing and re-adding the Foundation framework in combination with the above, but I still get the error.</p>
  
  <p>What's going on?</p>