private void button1_Click(object sender, EventArgs e)
{
    (new Form2()).Show();
}

private Form2 frmForm2 = null;
private void button1_Click(object sender, EventArgs e)
{
    if (frmForm2 == null || frmForm2.IsDisposed)
    {
        frmForm2 = new Form2();
        frmForm2.Show();
    }
    else
    {
        frmForm2.WindowState = FormWindowState.Normal;
        frmForm2.Activate();
    }
}