title: Drop duplicates while preserving NaNs in pandas
body: <p>When using the <code>drop_duplicates()</code> method I reduce duplicates but also merge all <code>NaNs</code> into one entry. How can I drop duplicates while preserving rows with an empty entry (like <code>np.nan, None or  ''</code>)?</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'col':['one','two',np.nan,np.nan,np.nan,'two','two']})
  
  Out[]: 
     col
  0  one
  1  two
  2  NaN
  3  NaN
  4  NaN
  5  two
  6  two
  
  
  df.drop_duplicates(['col'])
  
  Out[]: 
     col
  0  one
  1  two
  2  NaN
  </code></pre>