Nix flakes silently duplicate nixpkgs

When you compose Nix flakes, each input can silently pull in its own copy of nixpkgs unless you explicitly tell it to follow your top-level one.

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    some-flake.url = "github:someone/cool-flake";

    # without this, cool-flake brings its own nixpkgs
    some-flake.inputs.nixpkgs.follows = "nixpkgs";
  };
}

Without the follows line you end up with multiple full copies of nixpkgs in the Nix store. Gigabytes of duplication, slower rebuilds, bigger closures, and the classic “why is my /nix/store suddenly 30 GB bigger?” surprise.

This is the kind of thing that makes the difference between a flake that “works great” and one that “feels heavy.” If you’re publishing a flake meant for others to consume, documenting the follows pattern (or structuring your inputs so it’s obvious) saves every downstream user from hitting this silently.