diff --git a/CONTRIBUTORS b/CONTRIBUTORS
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -1,3 +1,4 @@
 Daniele Serafini
 Ishan Bhanuka
+Kumar Shivendu
 Quentin Campos
diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -192,6 +192,7 @@
         return summary
 
     def content_add(self, content: List[Content]) -> Dict:
+        content = list(set(content))
         contents = [attr.evolve(c, ctime=now()) for c in content]
         return self._content_add(list(contents), with_data=True)
 
@@ -354,6 +355,7 @@
                 yield {algo: content[algo] for algo in DEFAULT_ALGORITHMS}
 
     def directory_add(self, directories: List[Directory]) -> Dict:
+        directories = list(set(directories))
         # Filter out directories that are already inserted.
         missing = self.directory_missing([dir_.id for dir_ in directories])
         directories = [dir_ for dir_ in directories if dir_.id in missing]
@@ -479,6 +481,7 @@
 
     def revision_add(self, revisions: List[Revision]) -> Dict:
         # Filter-out revisions already in the database
+        revisions = list(set(revisions))
         missing = self.revision_missing([rev.id for rev in revisions])
         revisions = [rev for rev in revisions if rev.id in missing]
         self.journal_writer.revision_add(revisions)
@@ -589,10 +592,7 @@
         return revision.id
 
     def release_add(self, releases: List[Release]) -> Dict:
-        to_add = []
-        for rel in releases:
-            if rel not in to_add:
-                to_add.append(rel)
+        to_add = list(set(releases))
         missing = set(self.release_missing([rel.id for rel in to_add]))
         to_add = [rel for rel in to_add if rel.id in missing]
 
@@ -622,6 +622,7 @@
         return release.id
 
     def snapshot_add(self, snapshots: List[Snapshot]) -> Dict:
+        snapshots = list(set(snapshots))
         missing = self._cql_runner.snapshot_missing([snp.id for snp in snapshots])
         snapshots = [snp for snp in snapshots if snp.id in missing]
 
@@ -892,6 +893,7 @@
         )
 
     def origin_add(self, origins: List[Origin]) -> Dict[str, int]:
+        origins = list(set(origins))
         to_add = [ori for ori in origins if self.origin_get_one(ori.url) is None]
         # keep only one occurrence of each given origin while keeping the list
         # sorted as originally given