ทำไมเมื่อ Add ข้อมูลจาก Textbox ไปใส่ GridView แล้วข้อมูลที่เพิ่มเข้าไปใหม่ ไปทับข้อมูลเดิมค่ะ

	//*** TableRows (DataTable) ***//
	protected DataTable DataTableRows()
	{
        DataTable dt = new DataTable();
        DataRow dr;
		
		//*** Column ***//
        dt.Columns.Add("CustomerID");
        dt.Columns.Add("Name");
        dt.Columns.Add("Email");
        dt.Columns.Add("CountryCode");
        dt.Columns.Add("Budget");
        dt.Columns.Add("Used");
		
		//*** Rows ***//
		dr = dt.NewRow();
        dr["CustomerID"] = "C001";
        dr["Name"] = "Win Weerachai";
        dr["Email"] = "[email protected]";
        dr["CountryCode"] = "TH";
        dr["Budget"] = "1000000";
        dr["Used"] = "600000";
        dt.Rows.Add(dr);

		//*** Rows ***//
		dr = dt.NewRow();
        dr["CustomerID"] = "C002";
        dr["Name"] = "John Smith";
        dr["Email"] = "[email protected]";
        dr["CountryCode"] = "EN";
        dr["Budget"] = "2000000";
        dr["Used"] = "800000";
        dt.Rows.Add(dr);

		//*** Rows ***//
		dr = dt.NewRow();
        dr["CustomerID"] = "C003";
        dr["Name"] = "Jame Born";
        dr["Email"] = "[email protected]";
        dr["CountryCode"] = "US";
        dr["Budget"] = "3000000";
        dr["Used"] = "600000";
        dt.Rows.Add(dr);
		
		//*** Rows ***//
		dr = dt.NewRow();
        dr["CustomerID"] = "C004";
        dr["Name"] = "Chalee Angel";
        dr["Email"] = "[email protected]";
        dr["CountryCode"] = "US";
        dr["Budget"] = "4000000";
        dr["Used"] = "100000";
        dt.Rows.Add(dr);

		return dt; //*** Return DataTable ***//
	}

DataTable dtb = new DataTable();
DataRow dr = null;
dtb.Columns.Add(new DataColumn("index", typeof(int)));
        dtb.Columns.Add(new DataColumn("Description", typeof(string)));
        dtb.Columns.Add(new DataColumn("Quantity", typeof(int)));
        dtb.Columns.Add(new DataColumn("Unit", typeof(decimal)));
        dtb.Columns.Add(new DataColumn("Amount", typeof(decimal)));
        if (Session["table"] != null)
        {
            dtb = Session["table"] as DataTable;

            dr = dtb.NewRow();
            dr["Description"] = textDescrip.Text.ToString();
            dr["Quantity"] = textQuantity.Text.ToString();
            dr["Unit"] = textUnit.Text.ToString();
            dr["Amount"] = (Convert.ToInt32(textQuantity.Text)) * (Convert.ToDecimal(textUnit.Text));
            dtb.Rows.Add(dr);

            Session["table"] = dtb;
        }
        else
        {
            dr = dtb.NewRow();
            dr["Description"] = textDescrip.Text.ToString();
            dr["Quantity"] = textQuantity.Text.ToString();
            dr["Unit"] = textUnit.Text.ToString();
            dr["Amount"] = (Convert.ToInt32(textQuantity.Text)) * (Convert.ToDecimal(textUnit.Text));
            dtb.Rows.Add(dr);
            Session["table"] = dtb;
        }
        gvBillingcard.DataSource = dtb;
        gvBillingcard.DataBind();