title: Using underscore.js to find unique values based on dual properties
body: <p>Using Underscore.js, I'm trying to get unique values in an array that looks like this:</p>
  
  <pre><code>var links = [
      {source: 0, target: 1},
      {source: 0, target: 2},
      {source: 0, target: 3},
      {source: 0, target: 4},
      {source: 0, target: 1},
      {source: 4, target: 0}
  ];
  </code></pre>
  
  <p>so that it ends up like this:</p>
  
  <pre><code>var links = [
      {source: 0, target: 1},
      {source: 0, target: 2},
      {source: 0, target: 3},
      {source: 0, target: 4},
  ];
  </code></pre>
  
  <p>I want to get rid of links that have the same source &amp; target pair in the same order (0,1 == 0,1) of course, but also want to get rid of those that are the same but inverted (0,4 == 4,0).</p>
  
  <p>I'm sure I can do it with a double-nested _.map() but wanted to see if any Underscore magicians out there had a cleaner, more proper solution.</p>