title: Pandas: remove duplicates based on substring
body: <p>I have the following 2 columns, from a Pandas DataFrame:</p>
  <pre><code>antecedents        consequents
    apple               orange
    orange              apple
  
    apple               water
    apple               pineapple
  
    water               lemon
    lemon               water
  </code></pre>
  <p>I would like to remove duplicates that appear as bot antecedents and consequents, keeping only the first appearing, and thus obtain:</p>
  <pre><code>antecedents        consequents
    apple               orange
  
    apple               water
    apple               pineapple
  
    water               lemon
  </code></pre>
  <p>How can I achieve that using Pandas?</p>