title: Pandas: remove duplicate record while keeping its old value in dataframe for reference
body: <p>I am rewriting a piece of the old code using pandas. My data frame looks like this:</p>
  
  <pre><code>index stop_id   stop_name   stop_lat     stop_lon  stop_id2
  0         A12     Some St  40.889248   -73.898583      None
  1         A14     Some St  40.889758   -73.908573      None
  2         B09     Some St  40.788924   -74.846576      None
  3         A22     Some St  40.889248   -73.898583      None
  </code></pre>
  
  <p>Note that stop_lat and stop_lon are duplicated for stop_ids 'A12' and 'A22'.</p>
  
  <p>I want to remove the duplicate stop (stop_id='A22') while updating stop_d2 with the removed record's stop_id. So the data frame would look like this:</p>
  
  <pre><code>index stop_id   stop_name   stop_lat     stop_lon  stop_id2
  0         A12     Some St  40.889248   -73.898583      A22
  1         A14     Some St  40.889758   -73.908573      None
  2         B09     Some St  40.788924   -74.846576      None
  </code></pre>
  
  <p>Previously I have worked this task with keeping my data in dictionary:</p>
  
  <pre><code>d={'A12':['Some St', 40.889248, -73.898583, None],'A14': ['Some St', 40.889758, -73.908573, None],'B09':['Some St, 40.788924,-74.846576, None], 'A22':['Some St', 40.889248, -73.898583, None]}
  
  if d['A12'][1]+d['A12'][2]==d['A22'][1]+d['A22'][2]:
     del d['A22']
     d['A12'][-1]='A22'
  </code></pre>
  
  <p>I want to do similar task in pandas. I know if I just use:
      df=df.drop_duplicates(['stop_lat','stop_lon'])</p>
  
  <p>I will loose the duplicate record and won't retain its id.I need to keep id of the removed stop for proper metadata.</p>