>I want to prevent user to run or open the FORM twice,
>how to check the specific form are opened or active ?

One way is to define a custom property on the _VFP level:

ADDPROPERTY(_VFP, "MyForm", .NULL.)

In the Init event of the form add the following:

IF PEMSTATUS(_VFP, "MyForm", 5)
    IF ISNULL(_VFP.MyForm)
       _VFP.MyForm = thisform
    ELSE
       _VFP.MyForm.Show()
       RETURN .F.
    ENDIF
ELSE
    ADDPROPERTY(_VFP, "MyForm", thisform)
ENDIF

Now in the Destroy event of the form add the following:

_VFP.MyForm = .NULL.