There are rules for that, Unicode has standards (not only formal, but easily usable in most software libraries) for canonical forms that will collapse all the variations to a single representation.

But, of course, unicode can't define that the standard will cover only the canonical forms, and couldn't do that since the start, as it needed backwards compatibility with various pre-unicode encodings which had mutually incompatible principles, so it needed support for both composed and decomposed versions of the same characters.