fwiw nixpkgs has significantly more packages than arch (including the aur), and those packages are also significantly more up to date. No other repo comes close to nixpkgs in this regard. This has been the case for quite some years now.
How many of these are duplicates or just multiple versions though?
Repology already attempts to account for that, and Nixpkgs normal policy is one version per package unless there are specific reasons for it to be otherwise. Nixpkgs doesn't duplicate packages just to have alternative build options or whatever like the AUR does.
iiuc the “non unique” section on repology accounts for this. nixpkgs is almost twice the size of the aur if you count like this.
I also like that packages in Nixpkgs goes through code review.