My Solution,

1-give form name to the form with the same scx name for form

2-cerate property on the form named IsUnique with default value .T.

3-put below codes to the form load method

Local x,nFormCount
nFormCount=0
If This.IsUnique
	For x=1 To _Screen.FormCount
		If _Screen.Forms(x).Name==This.Name
			nFormCount=nFormCount+1
		Endif
		If nFormCount>1
			_Screen.Forms(x).Show() 
			Return .F.
		Endif
	Next
Endif

HTH