title: Increment column value in datatable if duplicate row is found
body: <p>I am trying to summarize some data in a data table, what I am attempting to do is to sum the total quantity of duplicate rows found.</p>
  
  <p>My data table looks like this.</p>
  
  <pre><code>|ForeName|SurName|Quantity|
  |Dave    |Smith  | 10000  |
  |Dave    |Smith  | 20000  |
  |Dave    |Smith  | 30000  |
  |John    |Peacock| 10000  |
  </code></pre>
  
  <p>I want to summarize this data to look like this.</p>
  
  <pre><code>|ForeName|SurName|Quantity|
  |Dave    |Smith  | 60000  |
  |John    |Peacock| 10000  |
  </code></pre>
  
  <p>At the moment I am searching for duplicates in the data table </p>
  
  <pre><code>Dim duplicates = From rows In dt.AsEnumerable().GroupBy(Function(r) New With {Key .a = r("ForeName"), Key .b = r("SurName")}).Where(Function(gr) gr.Count() &gt; 1).ToList()
  </code></pre>
  
  <p>However where to proceed from here I am unsure, has anyone ever came across a scenario like this and able to point me in the right direction.</p>