title: Remove rows with duplicate values
body: <p>I have to clean a table with duplicate rows:</p>
  
  <pre><code>id: serial id
  gid: group id
  url: string &lt;- this is the column that I have to cleanup
  </code></pre>
  
  <p>One <code>gid</code> may have multiple <code>url</code> values:</p>
  
  <pre><code>id    gid   url
  ----  ----  ------------
  1     12    www.gmail.com
  2     12    www.some.com
  3     12    www.some.com &lt;-- duplicate
  4     13    www.other.com
  5     13    www.milfsome.com &lt;-- not a duplicate
  </code></pre>
  
  <p>I want to execute one query against the entire table and delete all rows where the <code>gid</code> and <code>url</code> are duplicate. In the above sample, after the delete, I want to have only 1, 2, 4 and 5 remaining.</p>