title: Adding column values as long as a value in other column matches
body: <p>How do I add column values as long as a value in the other column is the same?</p>
  
  <p>e.g. from the following list:</p>
  
  <pre><code>Time   Value
  10       a
  20       b
  10       c
  10       d
  20       f
  </code></pre>
  
  <p>I want to obtain the following result:</p>
  
  <pre><code>Time      Value
  10       (a+c+d)
  20        (b+f)
  </code></pre>