title: Linq with group by in datatable
body: <p>I have a datatable with "year" and "month" columns</p>
  
  <pre><code>Year        Month
  2012        1
  2012        2
  2012        1
  </code></pre>
  
  <p>Now I want to check if there is any duplicated values, how can I do this? Thanks!</p>