diff --git a/sql/upgrades/156.sql b/sql/upgrades/156.sql
new file mode 100644
--- /dev/null
+++ b/sql/upgrades/156.sql
@@ -0,0 +1,25 @@
+-- SWH DB schema upgrade
+-- from_version: 155
+-- to_version: 156
+-- description: Make swh_release_add properly idempotent
+
+-- latest schema version
+insert into dbversion(version, release, description)
+      values(156, now(), 'Work In Progress');
+
+-- Create entries in release from tmp_release
+create or replace function swh_release_add()
+    returns void
+    language plpgsql
+as $$
+begin
+    perform swh_person_add_from_release();
+
+    insert into release (id, target, target_type, date, date_offset, date_neg_utc_offset, name, comment, author, synthetic)
+      select distinct t.id, t.target, t.target_type, t.date, t.date_offset, t.date_neg_utc_offset, t.name, t.comment, a.id, t.synthetic
+        from tmp_release t
+        left join person a on a.fullname = t.author_fullname
+        where not exists (select 1 from release where t.id = release.id);
+    return;
+end
+$$;
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
@@ -512,16 +512,20 @@
         return self._cql_runner.revision_get_random().id
 
     def release_add(self, releases: Iterable[Release]) -> Dict:
-        missing = self.release_missing([rel.id for rel in releases])
-        releases = [rel for rel in releases if rel.id in missing]
+        to_add = []
+        for rel in releases:
+            if rel not in to_add:
+                to_add.append(rel)
+        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]
 
-        self.journal_writer.release_add(releases)
+        self.journal_writer.release_add(to_add)
 
-        for release in releases:
+        for release in to_add:
             if release:
                 self._cql_runner.release_add_one(release_to_db(release))
 
-        return {"release:add": len(missing)}
+        return {"release:add": len(to_add)}
 
     def release_missing(self, releases):
         return self._cql_runner.release_missing(releases)
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -523,18 +523,19 @@
         return random.choice(list(self._revisions))
 
     def release_add(self, releases: Iterable[Release]) -> Dict:
-        releases = [rel for rel in releases if rel.id not in self._releases]
-        self.journal_writer.release_add(releases)
-
-        count = 0
+        to_add = []
         for rel in releases:
+            if rel.id not in self._releases and rel not in to_add:
+                to_add.append(rel)
+        self.journal_writer.release_add(to_add)
+
+        for rel in to_add:
             if rel.author:
                 self._person_add(rel.author)
             self._objects[rel.id].append(("release", rel.id))
             self._releases[rel.id] = rel
-            count += 1
 
-        return {"release:add": count}
+        return {"release:add": len(to_add)}
 
     def release_missing(self, releases):
         yield from (rel for rel in releases if rel not in self._releases)
diff --git a/swh/storage/sql/30-swh-schema.sql b/swh/storage/sql/30-swh-schema.sql
--- a/swh/storage/sql/30-swh-schema.sql
+++ b/swh/storage/sql/30-swh-schema.sql
@@ -17,7 +17,7 @@
 
 -- latest schema version
 insert into dbversion(version, release, description)
-      values(155, now(), 'Work In Progress');
+      values(156, now(), 'Work In Progress');
 
 -- a SHA1 checksum
 create domain sha1 as bytea check (length(value) = 20);
diff --git a/swh/storage/sql/40-swh-func.sql b/swh/storage/sql/40-swh-func.sql
--- a/swh/storage/sql/40-swh-func.sql
+++ b/swh/storage/sql/40-swh-func.sql
@@ -568,9 +568,10 @@
     perform swh_person_add_from_release();
 
     insert into release (id, target, target_type, date, date_offset, date_neg_utc_offset, name, comment, author, synthetic)
-    select t.id, t.target, t.target_type, t.date, t.date_offset, t.date_neg_utc_offset, t.name, t.comment, a.id, t.synthetic
-    from tmp_release t
-    left join person a on a.fullname = t.author_fullname;
+      select distinct t.id, t.target, t.target_type, t.date, t.date_offset, t.date_neg_utc_offset, t.name, t.comment, a.id, t.synthetic
+        from tmp_release t
+        left join person a on a.fullname = t.author_fullname
+        where not exists (select 1 from release where t.id = release.id);
     return;
 end
 $$;
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -1227,13 +1227,17 @@
             ("release", Release.from_dict(data.release))
         ]
 
-        actual_result = swh_storage.release_add([data.release, data.release2])
+        actual_result = swh_storage.release_add(
+            [data.release, data.release2, data.release, data.release2]
+        )
         assert actual_result == {"release:add": 1}
 
-        assert list(swh_storage.journal_writer.journal.objects) == [
-            ("release", Release.from_dict(data.release)),
-            ("release", Release.from_dict(data.release2)),
-        ]
+        assert set(swh_storage.journal_writer.journal.objects) == set(
+            [
+                ("release", Release.from_dict(data.release)),
+                ("release", Release.from_dict(data.release2)),
+            ]
+        )
 
     def test_release_add_name_clash(self, swh_storage):
         release1 = data.release.copy()