title: Python pandas: adding list of elements in a specific column to find all_elements
body: <p>Suppose I have a column using pandas.dataframe like so:</p>
  
  <pre><code>id  available_fruits  
  1   ['apple', 'banana']   
  1   []
  2   ['apple', 'tomato']
  1   ['banana']
  2   ['kiwi']
  </code></pre>
  
  <p>I want to create <code>all_available_fruits</code> list without duplicates, which should be <code>['apple', 'banana', 'kiwi', 'tomato']</code>.</p>
  
  <p>In other word, I want to add all elements in list, in pandas.dataframe column. How can I achieve this?</p>