A new window called twice after the mainForm, but first Form not being closed

Hi everyone, i have a problem:

Here is my case:

i already command a NewForm to be called everytime the user select that. The MainForm is being called when i run the program, and when i click "NewForm", the FirstNewForm being called and the MainForm is closed. But, when i click "NewForm" again in the FirstNewForm that i had created, it called the SecondNewForm, but the FirstNewForm is not being closed.

Here is my question:

How do i close the FirstNewForm? i mean, how do i close a CurrentForm whenever i called a NewForm?

Here is the image when i command a "NewForm" when the MainForm still in there (not being showed in here, because i can upload only 2 images), and MainForm is closed when FirstNewForm being called: (if you see in the taskbar, just one Selling System Program in there)

Here is the image when i command a "NewForm" when the FirstNewForm still in there, and FirstNewForm is not being closed when SecondNewForm being called: (if you see in the taskbar,there are two Selling System Programs in there, so the FirstNewForm is not being closed)

How do i hide and close the FirstNewForm?

Here is the code:

private void AddNewForm(object sender, EventArgs e)
        {
            this.Hide();

            Form newForm = new Form();

            AddObjects(sender, e, newForm);

            UpdateTextPosition(sender, e, newForm);

            newForm.Size = new Size(1360, 735);

            newForm.Text = "Selling System";

            newForm.FormBorderStyle = FormBorderStyle.Fixed3D;

            newForm.AutoScaleMode = AutoScaleMode.Font;

            newForm.AutoScroll = true;

            newForm.AutoSizeMode = AutoSizeMode.GrowAndShrink;

            newForm.StartPosition = FormStartPosition.CenterScreen;

            newForm.MaximizeBox = false;

            newForm.Controls.Add(label1);

            newForm.Controls.Add(label2);

            newForm.Controls.Add(label3);

            newForm.Controls.Add(label4);

            newForm.Controls.Add(label5);

            newForm.Controls.Add(label6);

            newForm.Controls.Add(label7);

            newForm.Controls.Add(menuStrip1);

            newForm.ShowDialog();

            this.Close();
        }

Thanks in advance! and sorry if i am post a long posting