title: Pandas `drop_duplicates` doesn't keep first row
body: <p>I created a Dataframe that has a duplicate row like the below:</p>
  
  <pre><code>df = pd.DataFrame({"Order Date": ["January 1, 2017", "March 15, 2017", "April 20, 2017", "June 23, 2017", "December 12, 2017", None, "April 20, 2017", "April 20, 2017"], 
           "Sales Person": ["John", "John", "Rick", "Mary", "Mary", "Rick", "Rick", "Rick"],
           "Items Sold": [4, -999, 1, np.nan, 7, 3, 1, 1],
           "Item Price": [4.99, 1.99, 9.99, 19.99, 0.99, 2.99, 9.99, 9.99]})
  </code></pre>
  
  <p>Which looks like this in Jupyter:
  <a href="https://i.stack.imgur.com/NURkn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NURkn.png" alt="Dataframe"></a></p>
  
  <p>If I get the duplicates it correctly shows the two rows that are duplicates.</p>
  
  <pre><code>df[df.duplicated()]
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/BFKtS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BFKtS.png" alt="Duplicates"></a></p>
  
  <p>I then call <code>drop_duplicates</code> to drop the second duplicate and keep the first.</p>
  
  <pre><code>df.drop_duplicates()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/CzJfK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CzJfK.png" alt="Dropped"></a></p>
  
  <p>However, it looks like it's removing both rows instead of keeping the first. Am I missing something in the <code>drop_duplicates</code> method? The docstring indicates that the <code>keep</code> parameter defaults to <code>first</code> and this still happens even if I explicitly put in that parameter.</p>