[Graham]

Most of us will work on a car without telling the world what it is so how is this is going to work.

No duplicate cars yes, i.e. same livery and number.

Even that is an issue, for the very reason you posted above 
Say I had bought a the same livery and car as you without either of us knowing until we declared our entries ... 
So far as I can recall duplicate liveries haven't happened yet in one of Grahams proxies, but at some point odds are, it will !

It does happen with other proxies, probably the reason that often those proxies allocate car numbers to entrants. My proxies tend to have a wider range of cars, so I have not bothered allocation of numbers as yet. But it is important to know what entrants intentions are in advance, so that I can spot any such concerns and resolve them. If I know there are going to be two identical cars I can ask one entrant to change the car number.