title: Combining rows in dataframe based on partial duplication
body: <p>I have a dataframe df in which some rows contain identical values, but one does not.</p>
  <pre><code>V1    V2    V3
  a     b     c
  a     b     d
  1     2     3
  1     2     4
  </code></pre>
  <p>I'd like to remove those rows which contain the same values for df$V1 and df$V2, and then combine the values in V3 using &quot;;;&quot; as a separator like so:</p>
  <pre><code>V1    V2    V3
  a     b     c;;d
  1     2     3;;4
  </code></pre>
  <p>Thanks!</p>