title: "ValueError: cannot reindex from a duplicate axis"
body: <p>I have the following df:</p>
  
  <pre><code>Timestamp                            A      B      C     ...     
  2014-11-09 00:00:00                     NaN     1      NaN   NaN      
  2014-11-09 00:00:00                      2     NaN     NaN   NaN             
  2014-11-09 00:00:00                     NaN    NaN     3     NaN   
  2014-11-09 08:24:00                     NaN    NaN     1     NaN         
  2014-11-09 08:24:00                     105    NaN     NaN   NaN           
  2014-11-09 09:19:00                     NaN    NaN     23    NaN          
  </code></pre>
  
  <p>And I would like to make the following:</p>
  
  <pre><code>Timestamp                            A      B      C     ...     
  2014-11-09 00:00:00                  2      1      3     NaN      
  2014-11-09 00:01:00                  NaN    NaN    NaN   NaN
  2014-11-09 00:02:00                  NaN    NaN    NaN   NaN
  ...                                  NaN    NaN    NaN   NaN
  2014-11-09 08:23:00                  NaN    NaN    NaN   NaN
  2014-11-09 08:24:00                  105    NaN     1    NaN         
  2014-11-09 08:25:00                  NaN    NaN     NaN  NaN     
  2014-11-09 08:26:00                  NaN    NaN     NaN  NaN
  2014-11-09 08:27:00                  NaN    NaN     NaN  NaN      
  ...                                  NaN    NaN     NaN  NaN      
  2014-11-09 09:18:00                  NaN    NaN     NaN  NaN  
  2014-11-09 09:19:00                  NaN    NaN     23   NaN      
  </code></pre>
  
  <p>That is: I would like to merge the columns with the same Timestamp (I have 17 columns), resample at 1 min granularity and for those column with no values I would like to have NaN.</p>
  
  <p>I started in the following ways:</p>
  
  <pre><code>df.groupby('Timestamp').sum()
  </code></pre>
  
  <p>and</p>
  
  <pre><code>df = df.resample('1Min', how='max')
  </code></pre>
  
  <p>but I obtained the following error:</p>
  
  <pre><code>ValueError: cannot reindex from a duplicate axis
  </code></pre>
  
  <p>How can I solve this problem? I'm just learning Python so I don't have experience at all.</p>
  
  <p>Thank you!</p>