validate grid view itemtemplate

Hi..

I'm using this tutioral

http://www.aspsnippets.com/Articles/Adding-Dynamic-Rows-in-GridView-with-DropDownLists-in-ASP.Net.aspx

Lets consider user slelect

Item1     Item2    Item3 on first row now user press add new row

In second Row if user select Item1 Item2 Item3 then how to show "Already Exists" alert message box..?

How to do so...?