title: Assigning unique id to duplicated rows
body: <p>If i have a data frame which looks like this:</p>
  
  <pre><code>x y
  13 a
  14 b
  15 c
  15 c
  14 b
  </code></pre>
  
  <p>and I wanted each group of equal rows to have a unique id, like this:</p>
  
  <pre><code>x y id
  13 a 1
  14 b 2
  15 c 3
  15 c 3
  14 b 2
  </code></pre>
  
  <p>Is there any easy way of doing this?</p>
  
  <p>Thanks</p>