title: Drop Duplicates isn't working on python
body: <p>I'm working in python, and have a dataframe(df), which includes the column 'CONTRACT_ID'. There are different numbers in this column. There is also another column called NPV, which is made up of numbers. </p>
  
  <pre><code> CONTRACT_ID                       NPV          
  0     1                            90                
  
  1     1                            90                   
  
  2     2                            130                     
  
  3     2                            130   
  
  4     3                            105
  
  5     3                            105
  ...
  
  ...
  </code></pre>
  
  <p>I don't want these duplicates to appear twice, so I tried to drop duplicates (only on column CONTRACT_ID) using this code:</p>
  
  <pre><code>df.drop_duplicates(subset=['CONTRACT_ID'], keep=False)
  </code></pre>
  
  <p>My expected result would be like so:</p>
  
  <pre><code>    CONTRACT_ID                    NPV                       
  0     1                            90                  
  
  1     2                            130                   
  
  2     3                            105                 
      ...
  </code></pre>
  
  <p>The code didn't work however. It ran with no error, but it simply didn't get rid of the duplicates at all. Am I doing something wrong?</p>
  
  <p>Thanks!</p>