Add uniqueness check for domains

modules/system/meta/domains.nix

@@ -0,0 +1,53 @@
+{
+  config,
+  self,
+  lib,
+  ...
+}:
+let
+  cfg = config.meta.domains;
+
+  duplicatedDomains =
+    self.nixosConfigurations
+    |> lib.mapAttrsToList (_: value: value.options.meta.domains.list.definitionsWithLocations)
+    |> lib.concatLists
+    |> lib.concatMap (
+      entry:
+      map (domain: {
+        file = entry.file;
+        inherit domain;
+      }) entry.value
+    )
+    |> lib.groupBy (entry: toString entry.domain)
+    |> lib.filterAttrs (domain: entries: lib.length entries > 1);
+
+  errorMessage =
+    duplicatedDomains
+    |> lib.mapAttrsToList (
+      domain: entries:
+      "Duplicate domain \"${domain}\" found in:\n"
+      + lib.concatMapStrings (entry: "  - ${entry.file}\n") entries
+    )
+    |> lib.concatStrings;
+in
+{
+  options.meta.domains = {
+    list = lib.mkOption {
+      type = lib.types.listOf lib.types.nonEmptyStr;
+      default = [ ];
+      internal = true;
+    };
+    assertUnique = lib.mkEnableOption "" // {
+      default = true;
+    };
+  };
+
+  config = lib.mkIf cfg.assertUnique {
+    assertions = [
+      {
+        assertion = duplicatedDomains == { };
+        message = errorMessage;
+      }
+    ];
+  };
+}