Are multiple versions of the same package in the module graph allowed?

[Peter Kriens]

It is clear that it is illegal for a module to be exposed to 2 or more modules that export the same package. I assume that such an application will not start and throw an exception? 

That said, there are many graphs were two or more packages are present in the graph, their modules are just never read by any module simultaneously. From reading the proposal I could not deduct if this is flagged as wrong or if this is actually allowed?