--- a/python/mozbuild/mozpack/packager/formats.py
+++ b/python/mozbuild/mozpack/packager/formats.py
@@ -6,16 +6,17 @@ from __future__ import absolute_import
 
 from mozpack.chrome.manifest import (
     Manifest,
     ManifestEntryWithRelPath,
     ManifestInterfaces,
     ManifestChrome,
     ManifestBinaryComponent,
     ManifestResource,
+    ManifestMultiContent,
 )
 from mozpack.errors import errors
 from urlparse import urlparse
 import mozpack.path as mozpath
 from mozpack.files import (
     ManifestFile,
     XPTFile,
 )
@@ -157,17 +158,21 @@ class FlatSubFormatter(object):
                 relbase = mozpath.basename(entry.base)
                 relpath = mozpath.join(relbase,
                                             mozpath.basename(path))
                 self.add_manifest(Manifest(parent, relpath))
             self.copier.add(path, ManifestFile(entry.base))
 
         if isinstance(entry, ManifestChrome):
             data = self._chrome_db.setdefault(entry.name, {})
-            entries = data.setdefault(entry.type, [])
+            if isinstance(entry, ManifestMultiContent):
+                entries = data.setdefault(entry.type, {}) \
+                              .setdefault(entry.id, [])
+            else:
+                entries = data.setdefault(entry.type, [])
             for e in entries:
                 # Ideally, we'd actually check whether entry.flags are more
                 # specific than e.flags, but in practice the following test
                 # is enough for now.
                 if not entry.flags or e.flags and entry.flags == e.flags:
                     errors.fatal('"%s" overrides "%s"' % (entry, e))
             entries.append(entry)
 

--- a/python/mozbuild/mozpack/test/test_packager_formats.py
+++ b/python/mozbuild/mozpack/test/test_packager_formats.py
@@ -14,16 +14,18 @@ from mozpack.files import (
     GeneratedFile,
     ManifestFile,
 )
 from mozpack.chrome.manifest import (
     ManifestContent,
     ManifestComponent,
     ManifestResource,
     ManifestBinaryComponent,
+    ManifestSkin,
+    ManifestLocale,
 )
 from mozpack.errors import (
     errors,
     ErrorMessage,
 )
 from mozpack.test.test_files import (
     MockDest,
     foo_xpt,
@@ -463,10 +465,39 @@ class TestFormatters(unittest.TestCase):
         self.assertEqual(e.exception.message,
             'Error: "content bar bar/unix" overrides '
             '"content bar bar/win os=WINNT"')
 
         # Adding something more specific still works.
         f.add_manifest(ManifestContent('chrome', 'bar', 'bar/win',
                                        'os=WINNT osversion>=7.0'))
 
+        # Variations of skin/locales are allowed.
+        f.add_manifest(ManifestSkin('chrome', 'foo', 'classic/1.0',
+                                    'foo/skin/classic/'))
+        f.add_manifest(ManifestSkin('chrome', 'foo', 'modern/1.0',
+                                    'foo/skin/modern/'))
+
+        f.add_manifest(ManifestLocale('chrome', 'foo', 'en-US',
+                                    'foo/locale/en-US/'))
+        f.add_manifest(ManifestLocale('chrome', 'foo', 'ja-JP',
+                                    'foo/locale/ja-JP/'))
+
+        # But same-skin/locale still error out.
+        with self.assertRaises(ErrorMessage) as e:
+            f.add_manifest(ManifestSkin('chrome', 'foo', 'classic/1.0',
+                                        'foo/skin/classic/foo'))
+
+        self.assertEqual(e.exception.message,
+            'Error: "skin foo classic/1.0 foo/skin/classic/foo" overrides '
+            '"skin foo classic/1.0 foo/skin/classic/"')
+
+        with self.assertRaises(ErrorMessage) as e:
+            f.add_manifest(ManifestLocale('chrome', 'foo', 'en-US',
+                                         'foo/locale/en-US/foo'))
+
+        self.assertEqual(e.exception.message,
+            'Error: "locale foo en-US foo/locale/en-US/foo" overrides '
+            '"locale foo en-US foo/locale/en-US/"')
+
+
 if __name__ == '__main__':
     mozunit.main()