diff --git a/swh/loader/package/storage.py b/swh/loader/package/storage.py
index 80aae46..77f69f5 100644
--- a/swh/loader/package/storage.py
+++ b/swh/loader/package/storage.py
@@ -1,96 +1,100 @@
 # Copyright (C) 2019 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 from swh.storage import get_storage
 
 from typing import Sequence, Dict, Set
 
 
 class ProxyStorage:
     def __init__(self, **storage):
         self.storage = get_storage(**storage)
-        self.contents_seen = set()
-        self.objects_seen = {'revision': set(), 'directory': set()}
+        self.objects_seen = {
+            'content': set(),    # set of content hashes (sha256) seen
+            'revision': set(),
+            'directory': set(),
+        }
 
     def _filter_missing_contents(
             self, content_hashes: Sequence[Dict]) -> Set[bytes]:
         """Return only the content keys missing from swh
 
         Args:
             content_hashes: List of sha256 to check for existence in swh
                 storage
 
         """
+        objects_seen = self.objects_seen['content']
         missing_hashes = []
         for hashes in content_hashes:
-            if hashes['sha256'] in self.contents_seen:
+            if hashes['sha256'] in objects_seen:
                 continue
-            self.contents_seen.add(hashes['sha256'])
+            objects_seen.add(hashes['sha256'])
             missing_hashes.append(hashes)
 
         return set(self.storage.content_missing(
             missing_hashes,
             key_hash='sha256',
         ))
 
     def content_add(self, content: Sequence[Dict]) -> Dict:
         contents = list(content)
         contents_to_add = self._filter_missing_contents(contents)
         return self.storage.content_add(
             x for x in contents if x['sha256'] in contents_to_add
         )
 
     def _filter_missing_ids(
             self,
             object_type: str,
             ids: Sequence[bytes]) -> Set[bytes]:
         """Filter missing ids from the storage for a given object type.
 
         Args:
             object_type: object type to use {revision, directory}
             ids: List of object_type ids
 
         Returns:
             Missing ids from the storage for object_type
 
         """
         objects_seen = self.objects_seen[object_type]
         missing_ids = []
         for id in ids:
             if id in objects_seen:
                 continue
             objects_seen.add(id)
             missing_ids.append(id)
 
         fn_by_object_type = {
             'revision': self.storage.revision_missing,
             'directory': self.storage.directory_missing,
         }
 
         fn = fn_by_object_type[object_type]
         return set(fn(missing_ids))
 
     def directory_add(self, directories: Sequence[Dict]) -> Dict:
         directories = list(directories)
         missing_ids = self._filter_missing_ids(
             'directory',
             (d['id'] for d in directories)
         )
         return self.storage.directory_add(
             d for d in directories if d['id'] in missing_ids
         )
 
     def revision_add(self, revisions):
         revisions = list(revisions)
         missing_ids = self._filter_missing_ids(
             'revision',
             (d['id'] for d in revisions)
         )
         return self.storage.revision_add(
             r for r in revisions if r['id'] in missing_ids
         )
 
     def __getattr__(self, key):
         return getattr(self.storage, key)