Don't duplicate setting

flake.nix

@@ -20,7 +20,11 @@
           libS = nixos-modules.lib { inherit lib config; };
         };
 
-        imports = (dir: map (file: ./. + "/${dir}/${file}") (ls dir)) "modules";
+        imports = (dir: map
+          (file: ./. + "/${dir}/${file}")
+          # assume that the nixos-modules repo is already imported if libS exists
+          (lib.remove (lib.optionalString (!args ? libs) "default.nix") (ls dir))
+        ) "modules";
       };
     };
 }