How to prevent duplicate user on signup page ?

//this is my code in registration form

public partial class Login : System.Web.UI.Page
{
    string cs = ConfigurationManager.ConnectionStrings["NDBCS"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {
        if(!IsPostBack)
        {
            LoadTrack();
        }

        
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(cs);
        con.Open();
        SqlCommand cmd = new SqlCommand("Insert into tbl_User (Firstname, Lastname, Track_id, Username, Email, Password) values (@firstname, @lastname, @track, @username, @email, @password)", con);
        cmd.Parameters.AddWithValue("@firstname", txtbox_FN.Text);
        cmd.Parameters.AddWithValue("@lastname", txtbox_LN.Text);
        cmd.Parameters.AddWithValue("@track", ddl_Track.SelectedValue);
        cmd.Parameters.AddWithValue("@username", txtbox_UN.Text);
        cmd.Parameters.AddWithValue("@email", txtbox_Email.Text);
        cmd.Parameters.AddWithValue("@password", txtbox_PW.Text);
        cmd.ExecuteNonQuery();
        con.Close();
    }

    private void LoadTrack()
    {
        try
        {
            using (SqlConnection con = new SqlConnection(cs))
            {
                SqlCommand cmd = new SqlCommand("select * from tbl_Track", con);
                con.Open();
                DataTable table = new DataTable();
                SqlDataAdapter adapter = new SqlDataAdapter(cmd);
                adapter.Fill(table);

                ddl_Track.DataSource = table;
                ddl_Track.DataValueField = "Track_id";
                ddl_Track.DataTextField = "Track_code";
                ddl_Track.DataBind();
                ddl_Track.Items.Insert(0, new ListItem("--Select Category--", "0"));
            }
        }

        catch (Exception ex)
        {

            Label1.ForeColor = System.Drawing.Color.Red;
            Label1.Text = "Something went wrong!." + ex.Message + "";
            Label1.Visible = false;
        }
    }