title: Getting error "The folder is already a source folder" when linking source in Eclipse
body: <p>In Eclipse, I am wanting to link an external source folder into my current project. To do so, I did the following steps:</p>
  
  <ol>
  <li>With the project open, go to the menu bar and choose Project -> Properties</li>
  <li>On the left side, choose "Java Build Path"</li>
  <li>In the source tab, click "Link Source..."</li>
  <li>Navigate to the other source folder (e.g. "OtherLocation/src")</li>
  </ol>
  
  <p>However, whenever I do this, I get the following error in the link source folder window:</p>
  
  <blockquote>
    <p>The folder is already a source folder.</p>
  </blockquote>
  
  <p>I'm really not sure what to make of this. I know that it is a source folder, that's why I'm trying to link to it!</p>
  
  <p>What is the real meaning of this error message and how can I go about resolving it?</p>