table is showing duplicate values while performing operations``

i am making an application in win forms in which there is a datagridview on the right and flields on the left when i enter some data gridview is not showing recently added record in a table

My Code is as follows

CRUD Class Code

{
    static string ConString = ConfigurationManager.ConnectionStrings["Constr"].ConnectionString;
    DataTable dt = new DataTable();
    SqlConnection con = new SqlConnection(ConString);
    SqlCommand cmd = new SqlCommand();

    #region----------> Select record
    public DataTable SelectRec()
    {
        string query = "select * from Users";

        //Users user = new Users();
        try {
            con.Open();
            cmd.Connection = con;
            cmd.CommandText = query;
            SqlDataAdapter sda = new SqlDataAdapter(cmd);
            sda.Fill(dt);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            con.Close();
        }
        return dt;
    }
    #endregion
    #region----------> Insert record

    public bool InsertRec(User u)
    {
        bool isOk = false;
        string query = "insert into Users(FirstName,LastName,Email,UserName,Password,Address,Contact,Gender,UserType,AddedDate,AddedBy)values(@FirstName,@LastName,@Email,@UserName,@Password,@Address,@Contact,@Gender,@UserType,@AddedDate,@AddedBy)";
        try
        {
            cmd.Connection = con;
            cmd.CommandText = query;
            cmd.Parameters.AddWithValue("@FirstName",u.FirstName);
            cmd.Parameters.AddWithValue("@LastName",u.LastName);
            cmd.Parameters.AddWithValue("@Email",u.Email);
            cmd.Parameters.AddWithValue("@UserName",u.UserName);
            cmd.Parameters.AddWithValue("@Password",u.Password);
            cmd.Parameters.AddWithValue("@Address",u.Address);
            cmd.Parameters.AddWithValue("@Contact",u.Contact);
            cmd.Parameters.AddWithValue("@Gender", u.Gender);
            cmd.Parameters.AddWithValue("@UserType",u.UserType);
            cmd.Parameters.AddWithValue("@AddedDate",u.AddedDate);
            cmd.Parameters.AddWithValue("@AddedBy",u.AddedBy);
            con.Open();
            //SqlDataAdapter sda = new SqlDataAdapter(cmd);
            //sda.Fill(dt);
            int run = cmd.ExecuteNonQuery();
            if (run > 0)
            {
                isOk = true;   
            }
            else
            {
                isOk = false;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            con.Close();
        }
        return isOk;
    }
    #endregion
    #region----------> Update record

    public bool UpdateRec(User u)
    {
        bool isOk = false;
        string query = "update Users set FirstName=@FirstName,LastName=@LastName,Email=@Email,UserName=@UserName,Password=@Password,Address=@Address,Contact=@Contact,Gender=@Gender,UserType=@UserType,AddedDate=@AddedDate,AddedBy=@AddedBy where Id=@Id";
        cmd.Connection = con;
        cmd.CommandText = query;
        try
        {
            cmd.Parameters.AddWithValue("@Id", u.Id);
            cmd.Parameters.AddWithValue("@FirstName", u.FirstName);
            cmd.Parameters.AddWithValue("@LastName", u.LastName);
            cmd.Parameters.AddWithValue("@Email", u.Email);
            cmd.Parameters.AddWithValue("@UserName", u.UserName);
            cmd.Parameters.AddWithValue("@Password", u.Password);
            cmd.Parameters.AddWithValue("@Address", u.Address);
            cmd.Parameters.AddWithValue("@Contact", u.Contact);
            cmd.Parameters.AddWithValue("@Gender", u.Gender);
            cmd.Parameters.AddWithValue("@UserType", u.UserType);
            cmd.Parameters.AddWithValue("@AddedDate", u.AddedDate);
            cmd.Parameters.AddWithValue("@AddedBy", u.AddedBy);
            con.Open();
            //SqlDataAdapter sda = new SqlDataAdapter(cmd);
            //sda.Fill(dt);
            int run = cmd.ExecuteNonQuery();
            if (run > 0)
            {
                isOk = true;
            }
            else
            {
                isOk = false;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            con.Close();
        }
        return isOk;
    }
    #endregion

    #region----------> Delete record
    public bool DeleteRec(User u)
    {
        bool isOk = false;
        string query = "delete from Users where Id=@Id";
        cmd.Connection = con;
        cmd.CommandText = query;
        cmd.Parameters.AddWithValue("@Id", u.Id);
        try
        {
            con.Open();
            int run = cmd.ExecuteNonQuery();
            if (run > 0)
            {
                isOk = true;
            }
            else
            {
                isOk = false;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            con.Close();
        }
        return isOk;
    }
    #endregion
    #region----------> other
    #endregion

}

On page load

private void Users_Load(object sender, EventArgs e)
{
    dataGridView1.DataSource = udl.SelectRec();    
}

Insert Snippet code - could also consider for update record just 

private void button2_Click(object sender, EventArgs e)
{
    int id  = Convert.ToInt32(txtuserid.Text);
    u.Id = id;
    u.FirstName = txtfn.Text;
    u.LastName = txtln.Text;
    u.Email = txtEmail.Text;
    u.UserName = txtun.Text;
    u.Password = txtpassword.Text;
    u.Address = txtaddress.Text;
    u.Contact = txtcontact.Text;
    u.Gender = cbg.Text;
    u.UserType = cbut.Text;
    u.AddedDate = DateTime.Now;
    // u.AddedBy = u.Id;
    bool success = udl.UpdateRec(u);
    if (success)
    {
        MessageBox.Show("Record Successfully Updated");
        //dataGridView1.Visible = false;
    }
    else
    {
        MessageBox.Show("Error occur during record update");
       
    }
    Clear();
    //dataGridView1.DataSource = udl.SelectRec();
    //dataGridView1.Update();
    //dataGridView1.Refresh();
    //dataGridView1.Visible = true;
}

Clear is a method to clear textbox values.

Help me out plaese a related  tutorial would be appreciable