ตรวจสอบว่าข้อมูลซ้ำรึป่าว ถ้าซ้ำไม่ให้แสดงแต่เพิ่มจำนวนแทน ใน Datagridview

string new_product = tbcode.Text;
            string sql = " SELECT Product_id, Product_name, PType_name, PCost_sell FROM Product_TB WHERE Product_id='" + new_product + "'";
            SqlCommand cmd = new SqlCommand(sql, conn);
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataTable dt;
            if (dvShow.DataSource != null)
            {
                dt = (DataTable)dvShow.DataSource;

            }
            else
            {
                dt = new DataTable();
            }
            adapter.Fill(dt);
            dvShow.DataSource = dt;
            conn.Close();
            

            if (dvShow.Columns.Count == 4)
            {
                dvShow.Columns.Add("", "จำนวนสินค้า");
            }
            for (int i = 0; i < dvShow.Rows.Count; i++)
            {
                dvShow.Rows[i].Cells[4].Value = "1";
            }

for (int i = 2; i < dvShow.Rows.Count; i++) 
            {
                for (int j = 0; j < i-1; j++) 
                {
                    if (dvShow.Rows[i].Cells[0].Value == dvShow.Rows[i].Cells[0].Value) 
                    {
                        dvShow.Rows[i].Cells[4].Value = +j;
                    }


                    if (dvShow.Rows[j].Cells[0].Value.ToString() == new_product)
                    {
                        dvShow.Rows.RemoveAt(j);
                    }
                }
            }