asp.net c# 为什么用户名可以重复注册，我的代码有什么问题
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;



namespace 网站
{
  public partial class register : System.Web.UI.Page
  {
  protected void Page_Load(object sender, EventArgs e)
  {

  }

  protected void btnSubmit_Click(object sender, EventArgs e)
  {
  string RUserName = this.RUserName.Text;
  string RPwd = this.RPwd.Text;
  string REmail = this.REmail.Text;
  string RCompany = this.RCompany.Text;
  string RAddress = this.RAddress.Text;
  string RStorey = this.RStorey.Text;
  string RHeight = this.RHeight.Text;
  string RArea = this.RArea.Text;
  string RFireRating = this.RFireRating.Text;
  string RPrincipal = this.RPrincipal.Text;
  string RPrincipalTel = this.RPrincipalTel.Text;
  string RCustodian = this.RCustodian.Text;
  string RCustodianTel = this.RCustodianTel.Text;
  string ROthers = this.ROthers.Text;
  string Sqlstr2 = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
  SqlConnection con = new SqlConnection(Sqlstr2);
  con.Open();
  string sql = @"insert into [cUser] (cName,cPassword,cEmail,cCompany,cAddress,cStorey,cHeight,cArea,
  cFireResistanceRating,cPrincipal,cPrincipalTel,cCustodian,cCustodianTel,cOtherInformation) values 
  ('" + RUserName + "','" + RPwd + "','" + REmail + "','" + RCompany + "','" + RAddress + "','" + RStorey + "','" + RHeight + "','" + RArea + "','" + RFireRating + "','" + RPrincipal + "','" + RPrincipalTel + "','" + RCustodian + "','" + RCustodianTel + "','" + ROthers + "')";
  SqlCommand cmd = new SqlCommand(sql, con);
  cmd.ExecuteNonQuery();
  Response.Write("<script type='text/javascript' language='javascript'>alert('注册成功！');</script>");
  con.Close();
  Server.Transfer("login.aspx");
  return; 

  }

  protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
  {
   
  string RUserName = args.Value;
  string Sqlstr3 = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
  SqlConnection con = new SqlConnection(Sqlstr3);
  con.Open();
  string sql1 = "select Count(*) from cUser where cName='" + RUserName + "'";
  SqlCommand cmd1 = new SqlCommand(sql1, con);
  int count = Convert.ToInt32(cmd1.ExecuteScalar());
  if (count > 0)
  {
  args.IsValid = false;

  }
  else
  {
  args.IsValid = true;
  }