Sorry for the previous question! The Form.B event is fired arter finish Form.A method.

So, you can use PROGRAM(1) at the form.B to identify if this event is fired from form.A. If it is, do not call to form.A method.

Example:

If Not ("A" $ Program(1))
thisform.A
EndIf

Note: Please, to make sure, check the sintax of PROGRAM