Hi, Saif.

>Is there any way to prevent users to open an already open form twice.
>I made command button disable after open a form. But when the form release this command button does not enable.

You can check in the form's Init whether there is another instance of the same form open, like:

llAlreadyExists = .f.
for each loForm in _Screen.Forms
   if loForm.Caption = this.Caption
      llAlreadyExists = .t.
      exit
   endif
next

return NOT llAlreadyExists

Notice that I'm using the Form's caption to check for the equivalence, but you can use some other property where we have some unique identification if you need it.

Hope this helps,