title: Check column for unique value
body: <p>My table:</p>
  
  <pre><code>id attribute
  1  2
  1  2
  2  3
  2  4
  5  1
  5  1
  6  3
  6  3
  6  5
  </code></pre>
  
  <p>Now I want only to output those <code>id</code> with <code>attribute</code>, if the attribute is the same for each <code>id</code>.</p>
  
  <p>In this sample table, the output would be</p>
  
  <pre><code>id attribute
  1  2
  5  1
  </code></pre>