And checking for ID collisions is generally extremely easy, both in code and compute.

And the ID collision was in user data which the air traffic system has to continuously accept during operations.

And it sounds like that data breaks down into individual flight plans - so it might be trivial to reject just one flight plan, and allow the rest to proceed.

BUT...doubtless the UK's flight control software came out of some multi-billion-pound government boondoggle. So we should be grateful that it doesn't crash planes into each other, or send innocent postal workers to jail for theft, and overlook these sorts of failures.