title: Add dataframe column that will represent a 'resetting index'
body: <p>This is what I'm looking for:</p>
  
  <p>Original DF:</p>
  
  <pre><code>    Col1   
  0   Dave
  1   Dave
  2   Mike
  3   Mike
  4   Mike
  5   Jeff
  </code></pre>
  
  <p>Desired DF:</p>
  
  <pre><code>    Col1   Col2
  0   Dave     1
  1   Dave     2
  2   Mike     1
  3   Mike     2
  4   Mike     3
  5   Jeff     1
  </code></pre>
  
  <p>Thanks for help!</p>