title: Replace all occurrences but first of a repeating string in a column using pandas
body: <p>I have a column, in a pandas dataframe, in which sometimes there is a repeating string:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th style="text-align: left;">col1</th>
  <th style="text-align: left;">col2</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td style="text-align: left;">1</td>
  <td style="text-align: left;">hello</td>
  </tr>
  <tr>
  <td style="text-align: left;">2</td>
  <td style="text-align: left;">bye</td>
  </tr>
  <tr>
  <td style="text-align: left;">3</td>
  <td style="text-align: left;">hello</td>
  </tr>
  <tr>
  <td style="text-align: left;">4</td>
  <td style="text-align: left;">morning</td>
  </tr>
  <tr>
  <td style="text-align: left;">5</td>
  <td style="text-align: left;">night</td>
  </tr>
  <tr>
  <td style="text-align: left;">6</td>
  <td style="text-align: left;">hello</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Would I would like to do is to modify all but the first occurence of &quot;hello&quot; in &quot;hello again&quot;. So the first occurence of hello remains the same.</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th style="text-align: left;">col1</th>
  <th style="text-align: left;">col2</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td style="text-align: left;">1</td>
  <td style="text-align: left;">hello</td>
  </tr>
  <tr>
  <td style="text-align: left;">2</td>
  <td style="text-align: left;">bye</td>
  </tr>
  <tr>
  <td style="text-align: left;">3</td>
  <td style="text-align: left;">hello again</td>
  </tr>
  <tr>
  <td style="text-align: left;">4</td>
  <td style="text-align: left;">morning</td>
  </tr>
  <tr>
  <td style="text-align: left;">5</td>
  <td style="text-align: left;">night</td>
  </tr>
  <tr>
  <td style="text-align: left;">6</td>
  <td style="text-align: left;">hello again</td>
  </tr>
  </tbody>
  </table>
  </div>