fix: avoid merging results of functor. dont use types.functionTo merge

checkmate/modules/tests/aspect_functor_merge.nix

@@ -0,0 +1,65 @@
+# Test that when multiple modules define the same aspect with a custom
+# __functor twice, the functor is not called once per definition (which would
+# duplicate the includes). Using lib.types.functionTo merges both functors.
+#
+# This was a bug where `functionTo` merge would
+# invoke all definitions and merge their results, causing duplication
+# when the functor uses `self` (the merged aspect) to produce includes.
+#
+# See https://github.com/vic/den/issues/216
+{ lib, new-scope, ... }:
+{
+
+  flake.tests."test functor merge does not duplicate includes" =
+    let
+      first = lib.evalModules {
+        modules = [
+          (new-scope "kit")
+          # Two separate modules defining the same aspect with same __functor.
+          {
+            kit.aspects.groups = {
+              myclass.names = [ "alice" ];
+              __functor = self: {
+                inherit (self) myclass;
+              };
+            };
+          }
+          {
+            kit.aspects.groups = {
+              myclass.names = [ "bob" ];
+              __functor = self: {
+                inherit (self) myclass;
+              };
+            };
+          }
+          (
+            { config, ... }:
+            {
+              kit.aspects.main = {
+                myclass.names = [ "main" ];
+                includes = [ config.kit.aspects.groups ];
+              };
+            }
+          )
+        ];
+      };
+
+      second = lib.evalModules {
+        modules = [
+          { options.names = lib.mkOption { type = lib.types.listOf lib.types.str; }; }
+          first.config.kit.modules.myclass.main
+        ];
+      };
+
+      expr = lib.sort (a: b: a < b) second.config.names;
+      expected = [
+        "alice"
+        "bob"
+        "main"
+      ];
+    in
+    {
+      inherit expr expected;
+    };
+
+}

nix/types.nix

@@ -24,6 +24,35 @@ let
       apply = fn;
     };
 
+  # Like lib.types.functionTo, but it does not merges all definitions, and keeps
+  # just the last one.
+  functorType = lib.mkOptionType {
+    name = "aspectFunctor";
+    description = "aspect functor function";
+    check = lib.isFunction;
+    merge =
+      loc: defs:
+      let
+        # Use only the last definition to avoid duplication from
+        # functionTo merging all definitions with the same args.
+        # All definitions receive the same merged `self`, so they
+        # produce equivalent results - picking one is correct.
+        lastDef = lib.last defs;
+        innerType = providerType;
+      in
+      {
+        __functionArgs = lib.functionArgs lastDef.value;
+        __functor =
+          _: callerArgs:
+          (lib.modules.mergeDefinitions (loc ++ [ "<function body>" ]) innerType [
+            {
+              inherit (lastDef) file;
+              value = lastDef.value callerArgs;
+            }
+          ]).mergedValue;
+      };
+  };
+
   # Check if function has submodule-style arguments
   isSubmoduleFn =
     m:
@@ -101,7 +130,7 @@ let
           internal = true;
           visible = false;
           description = "Functor to default provider";
-          type = lib.types.functionTo providerType;
+          type = functorType;
           default = aspect: { class, aspect-chain }: if true || (class aspect-chain) then aspect else aspect;
         };