title: How can I remove unique elements in a list? Haskell
body: <p>How would I remove unique elements in a list, so only frequently occurring elements remain, (only using prelude functions). for example</p>
  
  <pre><code>["abc","abc","a","b","c","b"]
  </code></pre>
  
  <p>would return:</p>
  
  <pre><code>["abc","b"]
  </code></pre>
  
  <p>In the case that all elements occur exactly once, all would be returned.</p>
  
  <p>If it helps, I have already implemented a sort function.</p>