This crossed my mind more than once, and let’s try to keep it at least a bit decent.

Is there a list of registrations which must absolutely not be issued, and who keeps track?

I would imagine that there is a list somewhere that has the details of registrations that must not be used…would be quite amusing to find ones that have been ‘abused’…lol!

I would also imagine that it would be down to the authorities/governing bodies of each country (in the UK, that would be the Civil Aviation Authority, CAA) as each aircraft has to been registered with them on the G-INFO database.