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The uniqueness of subjects is an interesting fact. It's the reason why we never have to explain why it's a good idea to merge all the reifiers of any given subject. It's also the reason why we never have to explain why it's a bad idea to merge the reifiers of different subjects. (Obviously, the resulting reifier would not reify a single subject, and there's no point in having a reifier for any number of subjects other than exactly one.Which brings us to the cardinal rule of the Topic Maps paradigm: every reifier must be understood as reifying exactly one subject, no more and no less.

Of course, what constitutes a subject, in any instance, is up to the application designer, and to the users of the application, to whatever extent the application designer delegates the necessary authority to the users.

Aside: All subjects (concepts, notions, ideas, things, persons, relationships, etc. etc.) are unique.