Check for Duplicate values while inserting in GridView without SQL in ASP.Net

Here Mudassar Ahmed Khan has explained with an example, how to check for duplicate values while inserting in GridView without SQL in ASP.Net using C# and VB.Net. When the Add Button is clicked, the values of TextBoxes will be compared with the Row values of the GridView using ASP.Net CustomValidator and JavaScript and if the values are duplicate, error messages will be displayed.