public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void showForm<T>(params object[] args)
            where T:Form
        {
            Form form = null;
            IEnumerable<T> forms = Application.OpenForms.OfType<T>();
            if (forms.Any())
                form = forms.First();
            if (form == null)
               form =(Form) Activator.CreateInstance(typeof(T), args);

            if (form.WindowState == FormWindowState.Minimized)
                form.WindowState = FormWindowState.Normal;
            else
                form.Show();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.showForm<Form2>();
        }
    }