Demo of an issue we just ran into

.gitignore

@@ -0,0 +1 @@
+/result

README.md

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+# Diamond dependencies with the nixpkgs module system
+
+This flake declares 2 `nixosConfigurations`:
+
+- `.#nixosConfigurations.use-module-binding`
+- `.#nixosConfigurations.use-module-filepath`
+
+They both include the `./vim-option.nix` module twice (simulating a "diamond
+dependency"), but in different ways:
+
+`use-module-filepath.nix`:
+```nix
+{
+  imports = [
+    ./vim-option.nix
+    ./vim-option.nix
+  ];
+
+  ...
+}
+```
+
+`use-module-binding.nix`:
+```nix
+let vim-option = import ./vim-option.nix;
+in
+{
+  imports = [
+    vim-option
+    vim-option
+  ];
+
+  ...
+}
+```
+
+This "double import via a binding" feels like something that's more likely to
+happen as the nix flake ecosystem evolves: someday you're going to end up using
+module A and module B where module B itself uses module A.
+
+Note that `.#nixosConfigurations.use-module-filepath` evaluates fine:
+
+```shell
+$ nix eval .#nixosConfigurations.use-module-filepath.config.system.build.toplevel
+trace: declaring _includeVim
+«derivation /nix/store/dmmxnrhn6c6yibzlcdlwjb1i5akdcjsi-nixos-system-nixos-24.11.20240923.30439d9.drv»
+```
+
+However, `.#nixosConfigurations.use-module-binding` fails to evaluate:
+
+```shell
+$ nix eval .#nixosConfigurations.use-module-binding.config.system.build.toplevel
+trace: declaring _includeVim
+trace: declaring _includeVim
+error:
+       … while calling the 'head' builtin
+         at /nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/lib/attrsets.nix:1575:11:
+         1574|         || pred here (elemAt values 1) (head values) then
+         1575|           head values
+             |           ^
+         1576|         else
+
+       … while evaluating the attribute 'value'
+         at /nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/lib/modules.nix:821:9:
+          820|     in warnDeprecation opt //
+          821|       { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
+             |         ^
+          822|         inherit (res.defsFinal') highestPrio;
+
+       … while evaluating the option `system.build.toplevel':
+
+       … while evaluating definitions from `/nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/nixos/modules/system/activation/top-level.nix':
+
+       … while evaluating the option `system.systemBuilderArgs':
+
+       … while evaluating definitions from `/nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/nixos/modules/system/activation/activatable-system.nix':
+
+       … while evaluating the option `system.activationScripts.etc.text':
+
+       … while evaluating definitions from `/nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/nixos/modules/system/etc/etc-activation.nix':
+
+       … while evaluating definitions from `/nix/store/p2hby44a0qzrnd1vxcpcgfav6160rmcv-source/nixos/modules/system/etc/etc.nix':
+
+       … while evaluating the option `environment.etc.dbus-1.source':
+
+       … while evaluating the option `environment.systemPackages':
+
+       … while evaluating definitions from `/nix/store/vxrd24hjs9k56cjfhchyy57zs0n8kg84-source/use-module-binding.nix':
+
+       (stack trace truncated; use '--show-trace' to show the full, detailed trace)
+
+       error: The option `_includeVim' in `/nix/store/vxrd24hjs9k56cjfhchyy57zs0n8kg84-source/use-module-binding.nix' is already declared in `/nix/store/vxrd24hjs9k56cjfhchyy57zs0n8kg84-source/use-module-binding.nix'.
+```

base.nix

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+{
+  nixpkgs.hostPlatform = "x86_64-linux";
+
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  system.stateVersion = "24.05";
+}

flake.nix

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+{
+  description = "A very basic flake";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+  };
+
+  outputs =
+    { nixpkgs, ... }:
+    {
+      nixosConfigurations.use-module-binding = nixpkgs.lib.nixosSystem {
+        modules = [
+          "${nixpkgs}/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix"
+          ./base.nix
+          ./use-module-binding.nix
+        ];
+      };
+      nixosConfigurations.use-module-filepath = nixpkgs.lib.nixosSystem {
+        modules = [
+          "${nixpkgs}/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix"
+          ./base.nix
+          ./use-module-filepath.nix
+        ];
+      };
+    };
+}

use-module-binding.nix

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+{
+  lib,
+  pkgs,
+  config,
+  ...
+}:
+let
+  vim-option = import ./vim-option.nix;
+in
+{
+  imports = [
+    vim-option
+    vim-option
+  ];
+
+  environment.systemPackages = lib.mkIf config._includeVim [ pkgs.vim ];
+}

use-module-filepath.nix

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+{
+  lib,
+  pkgs,
+  config,
+  ...
+}:
+{
+  imports = [
+    ./vim-option.nix
+    ./vim-option.nix
+  ];
+
+  environment.systemPackages = lib.mkIf config._includeVim [ pkgs.vim ];
+}

vim-option.nix

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+{ lib, ... }:
+{
+  options._includeVim = builtins.trace "declaring _includeVim" (
+    lib.mkOption {
+      internal = true;
+      default = false;
+    }
+  );
+
+  # Interestingly, this does not reproduce the same issue:
+  # options._includeVim = builtins.trace "declaring _includeVim" (lib.mkOption {internal = true;});
+  # config._includeVim = true;
+}