If a contact is in Trash (because they were merged) but the person with matching details fills in Membership what is a good outcome
I'd expect the Contact in Trash to be ignored by the dedupe rules, so if the dedupe rules don't match a (non-Trash) contact then a new one would be created.

If you add the Contact B .nz address as an extra email on Contact A then with the right dedupe rule you could avoid repeated new contacts.