18.—(1) Where an action on a claim (called in this section the firstmentioned proceedings) is commenced in a tribunal, no other civil proceedings relating to that claim (called in this section the second‑mentioned proceedings) may be commenced in any other court except —
(a)
where the second‑mentioned proceedings were commenced in that other court before the firstmentioned proceedings were commenced in the tribunal; or
(b)
where the claim before the tribunal is withdrawn, discontinued or abandoned, or is dismissed for lack of jurisdiction.
(2) No action for a claim may be brought before a tribunal if civil proceedings relating to that claim are pending in or have been heard and determined by any other court.