title: Pandas drop duplicates ignoring NaN
body: <p>In a Pandas df, I am trying to drop duplicates across multiple columns. Lots of data per row is <code>NaN</code>.</p>
  
  <p>This is only an example, the data is a mixed bag, so many different combinations exist.</p>
  
  <pre><code>df.drop_duplicates()
  
      IDnum       name            formNumber
  1   NaN         AP GROUP        028-11964
  2   1364615.0   AP GROUP        NaN
  3   NaN         AP GROUP        NaN
  </code></pre>
  
  <p>Hopeful Output:</p>
  
  <pre><code>    IDnum       name            formNumber
  1   1364615.0   AP GROUP        028-11964
  </code></pre>
  
  <p>EDIT:</p>
  
  <p>If the <code>df.drop_duplicates()</code> looks like this, would it change the solution? :</p>
  
  <pre><code>df.drop_duplicates()
  
      IDnum       name            formNumber
  0   NaN         AP GROUP        028-11964
  1   1364615.0   AP GROUP        028-11964
  2   1364615.0   AP GROUP        NaN
  3   NaN         AP GROUP        NaN
  </code></pre>