title: Using LINQ - Concat duplicate columns
body: <p>I have a list like:</p>
  
  <pre><code>ColumnA | ColumnB | ColumnC
  --------+---------+---------
  111     | 222     | xx
  111     | 222     | yy
  111     | 222     | zz
  </code></pre>
  
  <p>but I want to transfer as:</p>
  
  <pre><code>ColumnA | ColumnB | ColumnC
  --------+---------+---------
  111     | 222     | xx, yy, zz
  </code></pre>
  
  <p>If possible, I would like in the same list without creating more list to achieve the result using LINQ. Does anyone have a idea how to do this?</p>