title: How to merge two list columns when merging DataFrames?
body: <p>I have two <code>DataFrame</code>:  </p>
  
  <p><strong>df1:</strong>  </p>
  
  <pre><code>       date        ids
  0   2015-10-13       [978]
  1   2015-10-14  [978, 121]
  </code></pre>
  
  <p><strong>df2:</strong>  </p>
  
  <pre><code>       date        ids
  0   2015-10-13  [978, 12]
  1   2015-10-14     [2, 1]
  </code></pre>
  
  <p>When I merge them based on <code>date</code> as below:  </p>
  
  <pre><code>df = pandas.merge(df1, df2, on='date', sort=False)
  </code></pre>
  
  <p>I will have the below <code>DataFrame</code>:  </p>
  
  <pre><code>   date            ids_x             ids_y
  0   2015-10-13    [978]            [978, 12]
  1   2015-10-14    [978, 121]       [2, 1]
  </code></pre>
  
  <p>I want to have <strong>one</strong> <code>ids</code> column merged from both lists like <code>[978, 978, 12]</code> or preferably removing duplicates and have something like <code>[978, 12]</code>.</p>