public DataTable RemoveDuplicateRows(DataTable dTable, string colName)
{
   Hashtable hTable = new Hashtable();
   ArrayList duplicateList = new ArrayList();

   //Add list of all the unique item value to hashtable, which stores combination of key, value pair.
   //And add duplicate item value in arraylist.
   foreach (DataRow drow in dTable.Rows)
   {
      if (hTable.Contains(drow[colName]))
         duplicateList.Add(drow);
      else
         hTable.Add(drow[colName], string.Empty); 
   }

   //Removing a list of duplicate items from datatable.
   foreach (DataRow dRow in duplicateList)
      dTable.Rows.Remove(dRow);

   //Datatable which contains unique records will be return as output.
      return dTable;
}

protected void Button1_Click(object sender, EventArgs e)
{
   string strConn = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();
   SqlConnection conn = new SqlConnection(strConn);
   
   SqlDataAdapter da = new SqlDataAdapter("select * from emp", conn);
   DataSet ds = new DataSet();
   da.Fill(ds, "Emp");
   
   // Filling a employee table 
   DataTable dt = ds.Tables["Emp"];
   dt = RemoveDuplicateRows(dt, "empname");

   GridView1.DataSource = ds.Tables["Emp"].DefaultView;
   GridView1.DataBind();
}