title: Join two dataframe columns without repeated combinations
body: <p>I want to join the "item" column of  the two dataframes. </p>
  
  <pre><code>Dataframe1(df1)
  id    item
   1      1
   1      2 
   1      2
  
  Dataframe2(df2)
  _id    item
   44    1
   44    2
   44    2
  </code></pre>
  
  <p>I tried inner join, inorder to return the matching values</p>
  
  <pre><code> out= df1.join(df2,  df2.item==df1.item, 'inner')
   +---+----+----+---+
   | id|item|item|_id|
   +---+----+----+---+
   |  1| [1]| [1]| 44|
   |  1| [2]| [2]| 44|
   |  1| [2]| [2]| 44|
   |  1| [2]| [2]| 44|
   |  1| [2]| [2]| 44|
   +---+----+----+--- 
  </code></pre>
  
  <p>But i want the result like the below without including repeated matching combinations</p>
  
  <pre><code>+---+----+----+---+
   | id|item|item|_id|
   +---+----+----+---+
   |  1| [1]| [1]| 44|
   |  1| [2]| [2]| 44|
   |  1| [2]| [2]| 44|
   +---+----+----+---+
  </code></pre>
  
  <p>Another example could be </p>
  
  <pre><code>Dataframe1(df1)
  id    item
   1      1
   1      2 
   1      1
  
  Dataframe2(df2)
  _id    item
   44    1
   44    2
  </code></pre>
  
  <p>The result should be </p>
  
  <pre><code>+---+----+----+---+
  | id|item|item|_id|
   +---+----+----+---+
   |  1| [1]| [1]| 44|
   |  1| [2]| [2]| 44|
   +---+----+----+---+
  </code></pre>