title: How to avoid duplicate columns after join?
body: <p>I have two dataframes with the following columns: </p>
  
  <pre><code>df1.columns
  //  Array(ts, id, X1, X2)
  </code></pre>
  
  <p>and </p>
  
  <pre><code>df2.columns
  //  Array(ts, id, Y1, Y2)
  </code></pre>
  
  <p>After I do </p>
  
  <pre><code>val df_combined = df1.join(df2, Seq(ts,id))
  </code></pre>
  
  <p>I end up with the following columns: <code>Array(ts, id, X1, X2, ts, id, Y1, Y2)</code>. I could expect that the common columns would be dropped. Is there something that additional that needs to be done?</p>