(a) A former recovery or the pendency of a former action for the same cause of action between the same parties in the same or any other court having jurisdiction shall be a good cause of abatement. However, if the first action is so defective that no recovery can possibly be had, the pendency of a former action shall not abate the latter.

(b) Parol evidence shall be admissible to show that a matter apparently covered by the judgment was not passed upon by the court.