From C++ Windows Forms

[C++ Programming]

I'm tring from form1 to click on a button to open another form2
ex:Form1:
private: System::Void button1_Click(System::Object * sender, System::EventArgs * e)
{
TestForm *f= new TestForm();
f->Show();
}

when i press this button again the same form2 will open again.

How to prevent this? any solution please.