title: Spark structured streaming drop duplicates keep last
body: <p>I would like to maintain a streaming dataframe that get "update".</p>
  
  <p>To do so I will use <code>dropDuplicates</code>.</p>
  
  <p>But <code>dropDuplicates</code> drop the latest change.</p>
  
  <p>How can I retain the last only?</p>