. Please Help, How Can I Make My Codes Not To Register Two Users.

public partial class Registration : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LearningConnectionString"].ConnectionString);
            conn.Open();
            string checkuser = "select count (*) from UserData where UserName='" + TextBoxUserName.Text + "'";
            SqlCommand com = new SqlCommand(checkuser, conn);
            int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
            conn.Close();
            if (temp >= 1)
            {
                Response.Write("UserName already exists");
            }

        }
             
    }
    protected void Button_SignUp_Click(object sender, EventArgs e)
    {
        try
        {
            Guid NewGuid = Guid.NewGuid();

            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LearningConnectionString"].ConnectionString);
            conn.Open();
            string insertQuery = "insert into UserData(ID,Name,LastName,UserName,Email,Password,Gender,country,DateOfBirth) values (@ID ,@name ,@lastname ,@username ,@email ,@password ,@gender ,@country ,@dateofbirth)";
            SqlCommand com = new SqlCommand(insertQuery, conn);

            com.Parameters.AddWithValue("@ID", NewGuid.ToString());
            com.Parameters.AddWithValue("@name", TextBoxName.Text);
            com.Parameters.AddWithValue("@lastname", TextBoxLastName.Text);
            com.Parameters.AddWithValue("@username", TextBoxUserName.Text);
            com.Parameters.AddWithValue("@email", TextBoxEmail.Text);
            com.Parameters.AddWithValue("@password", TextBoxPassword.Text);
            com.Parameters.AddWithValue("@gender", DropDownListGender.SelectedItem.ToString());
            com.Parameters.AddWithValue("@country", DropDownListCountry.SelectedItem.ToString());
            com.Parameters.AddWithValue("@dateofbirth", TextBox_DOB.Text);

            com.ExecuteNonQuery();
            Response.Write("Your registration is successful");
            Response.Redirect("Login.aspx");

            conn.Close();
        }

        catch (Exception ex)
        {
            Response.Write("Error:" + ex.ToString());
        }
    
    }

}