C# ถ้าใส่วันกับเวลาในวันจองแล้วชนกันให้ขึ้ย Alert ไม่สามารถจองทับได้

if (txtPassword.Text.Length < 7)
            {
                lblStatus.Text = "กรุณากรอกรหัสผ่านให้ครบ 7 ตัวอักษร";
                lblStatus.ForeColor = System.Drawing.Color.Red;

            }
            else
            {
                int intNumRows = 0;
                String strSQL;
                strSQL = "SELECT COUNT(*) FROM Organizer WHERE username = '" + txtUsername.Text + "'" ;
                intNumRows = Convert.ToInt32(clsDB.QueryExecuteScalar(strSQL));

                if (intNumRows > 0)
                {
                    lblStatus.Text = "มี username ซ้ำในฐานข้อมูล";
                    lblStatus.ForeColor = System.Drawing.Color.Red;

                }
                else
                {
                    try
                    {
                        DataTable dt = new DataTable();

                        DateTime dts = Convert.ToDateTime(txtStartDate.Text);
                        DateTime dte = Convert.ToDateTime(txtEndDate.Text);


                        ADMange AD = new ADMange(this.AD_Host, this.AD_Base, this.AD_Port, this.AD_UserObjectClass, this.AD_authenticationWith, this.AD_Username, this.AD_Password);

                        if (AD.createUserAccount(txtUsername.Text, txtPassword.Text, dte.ToString("yyyy-MM-dd"), txtOrganizer.Text, "", "", "", "", "ou=Organizer,"))
                        {
                            AD.setAccountExpire(txtUsername.Text, Convert.ToDateTime(txtEndDate.Text));

                             strSQL = "INSERT INTO Organizer(username,password,orgname,eventname,start_date,end_date,start_time,end_time,flag,picture)" +
                                "VALUES" +
                                "('" + txtUsername.Text + "','" + txtPassword.Text + "','" + txtOrganizer.Text + "','' " +
                                " ,'" + dts.ToString("yyyy-MM-dd") + "','" + dte.ToString("yyyy-MM-dd") + "','"+ddlHourStart.SelectedItem+"','"+ddlHourStop.SelectedItem+"','enable','' )";

                             if (clsDB.QueryExecuteNonQuery(strSQL))
                             {

                                 Response.Write("<Script language=javascript> alert('ระบบทำการจองให้คุณเรียบร้อยแล้วค่ะ')</Script>");
                               
                             }

                        }
                        else
                        {

                            lblStatus.Text = "มี User นี้ในระบบ AD แล้ว";
                            lblStatus.ForeColor = System.Drawing.Color.Red;
                        }

                            txtUsername.Text = "";
                            txtOrganizer.Text = "";
                            txtStartDate.Text = "";
                            txtEndDate.Text = "";
                            ddlHourStart.Text = "";
                            ddlHourStop.Text = "";
                    }
                    catch (Exception ex)
                    {

                        lblStatus.Text = "";
                        lblStatus.ForeColor = System.Drawing.Color.Red;
                    }

                }
            }