How Can I Add The Stock And Totalprice Whenever Their Is Duplicate In A Product?

private void btnSaveOut_Click(object sender, EventArgs e)
        {
            double tps = Convert.ToDouble(txtStockOut.Text) * Convert.ToDouble(txtPriceOut.Text);
            try
            {
                MySqlConnection con = new MySqlConnection("Server=localhost;UID=root;Database=db_cignal");
                con.Open();
                MySqlCommand command = con.CreateCommand();
                command.CommandText = "update inventoryOut set Model=@Model,Stock=@Stock,Date=@Date,Price=@Price,TotalPrice=@Total where Product=@Product";
                command.Parameters.AddWithValue("@Product", cbProductOut.SelectedItem.ToString());
                command.Parameters.AddWithValue("@Model", txtModelOut.Text);
                command.Parameters.AddWithValue("@Stock", txtStockOut.Text);
                command.Parameters.AddWithValue("@Date", DateTime.Now);
                command.Parameters.AddWithValue("@Price", txtPriceOut.Text);
                command.Parameters.AddWithValue("@Total", tps.ToString());
                command.Prepare();
                command.ExecuteNonQuery();
                BindGridInventoryOut();
                ClearFieldsinInventoryOUT();
                con.Close();
                MessageBox.Show("Saved!");
            }
            catch (Exception r)
            {
                MessageBox.Show(r.ToString());
            }
        }