title: check combinations of columns in a DF to return unique rows
body: <pre><code>for a, b in itertools.combinations(number_of_notes_cols, 2):
      weekly_meetings_difference = all_meetings_data[(all_meetings_data[a] != all_meetings_data[b]) == True]
  </code></pre>
  
  <p>The code above used to work: it would return all the rows of all the combinations of pairs of weekly_meetings_difference's columns where the column values (if this was true for any pair of columns). Now, returning weekly_meetings_difference gives me some, but not all, of the rows where the column values changed.</p>
  
  <hr>
  
  <p>Edit with some code:</p>
  
  <p>Before (when everything seemed to be working fine):</p>
  
  <pre><code>Number of Notes 03112016    Number of Notes 03192016    Number of Notes 03272016    Number of Notes 04042016
  Meeting Name                
  X      12.0 NaN NaN NaN
  Y       5.0 5.0 NaN NaN
  Z       2.0 NaN NaN NaN
  W       NaN 6.0 713.0 740.0
  </code></pre>
  
  <p>After (now that I've updated the original dataframe from which I want information):</p>
  
  <pre><code>Number of Notes 03112016    Number of Notes 03192016    Number of Notes 03272016    Number of Notes 04042016    Number of Notes 04122016    Emails 04122016
  Meeting Name                        
  A   37.0 37.0 38.0 38.0 37.0
  X   12.0 NaN NaN NaN NaN NaN
  Y   5.0  5.0 NaN NaN NaN NaN
  Z   2.0  NaN NaN NaN NaN NaN
  </code></pre>
  
  <p>Now that I've done this edit, I am noticing row A was added after adding the extra column to the dataframe as well as row W being removed (they both should show each time)</p>