title: Python: get a frequency count based on two columns (variables) in pandas dataframe some row appers
body: <p>Hello I have the following dataframe.   </p>
  
  <pre><code>    Group           Size
  
      Short          Small
      Short          Small
      Moderate       Medium
      Moderate       Small
      Tall           Large
  </code></pre>
  
  <p>I want to count the frequency of how many time the same row appears in the dataframe.</p>
  
  <pre><code>    Group           Size      Time
  
      Short          Small        2
      Moderate       Medium       1 
      Moderate       Small        1
      Tall           Large        1
  </code></pre>