Not repeating the others:
If a package does end up using multiple version of the same lib, nixos can even deduplicate it somewhat, by symlinking one’s documentations to the other’s.
There's an optimzation I'm aware of where nix will de-duplicate individual files with hard links. This wouldn't be safe in almost any other context, but is safe with nix because the files are immutable. So you have have 100 variations of libcurl and only "pay" for the unique files.