title: How to avoid repetition in prolog
body: <p>I get several results with the same outputs. </p>
  
  <p>I want to get <code>jack,jane</code> only one time, but I get it two times. </p>
  
  <p>How can I avoid this? I tried <code>setof</code> but I couldn't get it done. Please can someone help me on this?</p>