Open Form only one Time

Question

Hello All,

I open several Forms in one main Form...
But my problem is that an allready opened Form is opening the second... third... fourth... time if I click on that button.
How it is possible to void a new opening of a allready opened Form?
and If possible how can I sent a warning, that the Form is allready open...?

thank you in advance

my procedure to open a form

SimFrm gsm = new SimFrm(this);
gsm.TopLevel = False;
this.Controls.Add(gsm);
gsm.Show();