RTF is a global modification that replaces all towns in the game.

Well, in any case, something will be duplicated because it's not very desirable to combine Light and Darkness, but otherwise, the variety is good, and it turned out to be quite interesting; many people like it.

You can't avoid duplication in any case; there are only 6 possible combinations of pairs, and 8 races.
As for Light and Darkness in one set, it's the most harmonious and atmospheric combination, perfectly fitting the concept of the old Humans, and also the new races; such a powerful combination could be useful. It's like "on the verge of balance between light and darkness."

Humans - Light - it's all clear here. Darkness is out of the question.

I understand, but depriving Humans of their legitimate, native Darkness is simply a cut, a simplification of the race within the framework of "only white, nothing black."

That is:
Naga Light + Summoning
Humans Light + Darkness
Dark Elves Darkness + Chaos
Orcs either Chaos, or Darkness + something (for example, Chaos + Summoning)
Lizardmen either Darkness, or Chaos + something (for example, Darkness + Summoning)