多次将验证码发送到电子邮件导致错误消息

我使用 c# 和 asp.net ，当用户创建新帐户时，将输入电子邮件和国家 ID 作为主键，然后如果用户单击 (VIRIFY ME) 按钮，程序将将此信息存储在 sql server 中，并向用户发送电子邮件消息，但我的问题是，当用户单击多个“更改我”按钮时，它会打印（电子邮件已存在）

   protected void Button1_Click2(object sender, EventArgs e)
{



    LabelErrorMSG.Text = "";
    String email = emailtextbox0.Text.Trim();
    String notionalID = textbox_National_ID.Text.Trim();


    try
    {
        if (notionalID != "" && email != "" && counter==1)
        {
            // insert notional ID and email into database 
            getdataobj.PageSignUpInsert(notionalID, email);

            /////////////////////////////////////////////////////////////////////
            conn.Open();

            //Generate Verification Code
            String code = getdataobj.GetRandomNumber().ToString();

            // Set Verification Code in database
            SqlCommand comm = new SqlCommand("UPDATE trained SET VerificationCode='" + code + "' where NationalID='" + notionalID + "'", conn);
            comm.ExecuteNonQuery();
            conn.Close();

            //Send Email to the user with Verification Code
            SmtpClient smtpClient = new SmtpClient();
            MailMessage mailMessage = new MailMessage("[email protected]", email, "", "");
            mailMessage.To.Add(new MailAddress(email));
            mailMessage.Subject = "Saudia Summer Traning";
            mailMessage.Body = code;
            smtpClient.EnableSsl = true;
            smtpClient.Send(mailMessage);

            Panel1.Visible = true;
        }
        else
        {
            LabelErrorMSG.Text = "you must insert national ID and email ";
        }
     ////////////////////////////////////////////////////////////////////
    }

    catch (SqlException ex)
    {
        if (ex.Number == 2627)
        {
            if (ex.Message.Contains("UNIQUE"))
            {

                ///error msg regarding Unique key violation.
                LabelErrorMSG.Text = "The email already exist ";
            }
            if (ex.Message.Contains("PRIMARY"))
            {
                //error msg regarding Primary key violation.
                LabelErrorMSG.Text = "The national ID already exist ";
            }

        }
    }

}