เasp.net C# ช็คข้อมูลจาก database แล้วจะโชว่เมื่อใส่ข้อมูลแล้วซ้ำได้อย่างไรคะ

 private int CheckMail (string Email )
    {
        using (SqlConnection cn = new SqlConnection(Globals.Con.ToString()))
        {
            string sql = "SELECT tbx_AuthFucnc.AuthEmail, tbx_AuthFucnc.FunctID, ";
            sql = sql + " tbx_TranKeyContract.CompanyID,tbx_TranKeyContract.[TranKeyContractID]";
            sql = sql + "FROM tbx_AuthFucnc INNER JOIN  tbx_TranKeyContract ON tbx_AuthFucnc.TranKeyContractID = tbx_TranKeyContract.TranKeyContractID";
            sql = sql + " WHERE (tbx_AuthFucnc.AuthEmail = N'[email protected]')";
            sql = sql + "AND (tbx_AuthFucnc.FunctID = 1)AND (tbx_TranKeyContract.CompanyID = 712)";

            SqlCommand cmd = new SqlCommand(sql, cn);
            cmd.CommandType = CommandType.Text;


            cn.Open();

            DataTable dt = new DataTable();

            System.Data.Common.DbDataReader dr;
            dr = cmd.ExecuteReader();

            dt.Load(dr);

            int ret = 0;
            if (dt.Rows.Count > 0)
            {
                ret = dt.Rows[0][0].ToString().ToInt();
            }

            return ret;

        }
    }


                if (CheckMail(txtPromoMAil.Text,funcID) > 0)
                {
                    Alert("Err", "Dupplicate Promotion Mail", true);
                    return;
                }
             

            }