title: Pandas drop duplicates on elements made of lists
body: <p>Say my dataframe is:</p>
  
  <pre><code>df = pandas.DataFrame([[[1,0]],[[0,0]],[[1,0]]])
  </code></pre>
  
  <p>which yields:</p>
  
  <pre><code>        0
  0  [1, 0]
  1  [0, 0]
  2  [1, 0]
  </code></pre>
  
  <p>I want to drop duplicates, and only get elements [1,0] and [0,0], if I write:</p>
  
  <pre><code>df.drop_duplicates()
  </code></pre>
  
  <p>I get the following error: TypeError: unhashable type: 'list'</p>
  
  <p>How can I call drop_duplicates()?</p>
  
  <p>More in general:</p>
  
  <pre><code>df = pandas.DataFrame([[[1,0],"a"],[[0,0],"b"],[[1,0],"c"]], columns=["list", "letter"])
  </code></pre>
  
  <p>And I want to call df["list"].drop_duplicates(), so drop_duplicates applies to a Series and not a dataframe?</p>