diff --git a/swh/indexer/storage/__init__.py b/swh/indexer/storage/__init__.py
--- a/swh/indexer/storage/__init__.py
+++ b/swh/indexer/storage/__init__.py
@@ -49,6 +49,34 @@
     return IndexerStorage(**args)
 
 
+def _check_duplicates(data, key):
+    """
+    If any two dictionaries in `data` have the same value for the
+    key, raises a `ValueError`.
+
+    Values associated to the key must be hashable.
+
+    Args:
+        data (List[dict]): List of dictionaries to be inserted
+        key (str): Name of the key that acts as id.
+
+    >>> _check_duplicates([
+    ...     {'id': 'foo', 'data': 'spam'},
+    ...     {'id': 'bar', 'data': 'egg'},
+    ... ], 'id')
+    >>> _check_duplicates([
+    ...     {'id': 'foo', 'data': 'spam'},
+    ...     {'id': 'foo', 'data': 'egg'},
+    ... ], 'id')
+    Traceback (most recent call last):
+      ...
+    ValueError: The same id is present more than once.
+    """
+    if len({item[key] for item in data}) < len(data):
+        raise ValueError(
+            'The same {} is present more than once.'.format(key))
+
+
 class IndexerStorage:
     """SWH Indexer Storage
 
@@ -216,6 +244,7 @@
                 default)
 
         """
+        _check_duplicates(mimetypes, 'id')
         db.mktemp_content_mimetype(cur)
         db.copy_to(mimetypes, 'tmp_content_mimetype',
                    ['id', 'mimetype', 'encoding', 'indexer_configuration_id'],
@@ -300,6 +329,7 @@
                 default)
 
         """
+        _check_duplicates(languages, 'id')
         db.mktemp_content_language(cur)
         # empty language is mapped to 'unknown'
         db.copy_to(
@@ -369,6 +399,8 @@
                   line, lang
 
         """
+        _check_duplicates(ctags, 'id')
+
         def _convert_ctags(__ctags):
             """Convert ctags dict to list of ctags.
 
@@ -449,7 +481,7 @@
             list: content_license entries which failed due to unknown licenses
 
         """
-        # Then, we add the correct ones
+        _check_duplicates(licenses, 'id')
         db.mktemp_content_fossology_license(cur)
         db.copy_to(
             ({
@@ -547,6 +579,8 @@
                 or skip duplicates (false, the default)
 
         """
+        _check_duplicates(metadata, 'id')
+
         db.mktemp_content_metadata(cur)
 
         db.copy_to(metadata, 'tmp_content_metadata',
@@ -614,6 +648,8 @@
               or skip duplicates (false, the default)
 
         """
+        _check_duplicates(metadata, 'id')
+
         db.mktemp_revision_metadata(cur)
 
         db.copy_to(metadata, 'tmp_revision_metadata',
@@ -666,6 +702,8 @@
               or skip duplicates (false, the default)
 
         """
+        _check_duplicates(metadata, 'origin_id')
+
         db.mktemp_origin_intrinsic_metadata(cur)
 
         db.copy_to(metadata, 'tmp_origin_intrinsic_metadata',
diff --git a/swh/indexer/storage/in_memory.py b/swh/indexer/storage/in_memory.py
--- a/swh/indexer/storage/in_memory.py
+++ b/swh/indexer/storage/in_memory.py
@@ -130,6 +130,10 @@
               (true) or skip duplicates (false)
 
         """
+        data = list(data)
+        if len({x['id'] for x in data}) < len(data):
+            # For "exception-compatibility" with the pgsql backend
+            raise ValueError('The same id is present more than once.')
         for item in data:
             item = item.copy()
             tool_id = item.pop('indexer_configuration_id')
diff --git a/swh/indexer/tests/storage/test_storage.py b/swh/indexer/tests/storage/test_storage.py
--- a/swh/indexer/tests/storage/test_storage.py
+++ b/swh/indexer/tests/storage/test_storage.py
@@ -223,6 +223,41 @@
         # data did change as the v2 was used to overwrite v1
         self.assertEqual(actual_data, expected_data_v2)
 
+    def add__duplicate_twice(self):
+        # given
+        tool_id = self.tools[tool_name]['id']
+
+        data_rev1 = {
+            'id': self.revision_id_2,
+            **example_data1,
+            'indexer_configuration_id': tool_id
+        }
+
+        data_rev2 = {
+            'id': self.revision_id_2,
+            **example_data2,
+            'indexer_configuration_id': tool_id
+        }
+
+        # when
+        endpoint(self, 'add')([data_rev1])
+
+        with self.assertRaises(ValueError):
+            endpoint(self, 'add')(
+                [data_rev2, data_rev2],
+                conflict_update=True)
+
+        # then
+        actual_data = list(endpoint(self, 'get')(
+            [self.revision_id_2, self.revision_id_1]))
+
+        expected_data = [{
+            'id': self.revision_id_2,
+            **example_data1,
+            'tool': self.tools[tool_name]
+        }]
+        self.assertEqual(actual_data, expected_data)
+
     @rename
     def get(self):
         # given
@@ -255,6 +290,7 @@
         missing,
         add__drop_duplicate,
         add__update_in_place_duplicate,
+        add__duplicate_twice,
         get,
     )
 
@@ -300,6 +336,7 @@
         test_content_mimetype_missing,
         test_content_mimetype_add__drop_duplicate,
         test_content_mimetype_add__update_in_place_duplicate,
+        test_content_mimetype_add__duplicate_twice,
         test_content_mimetype_get,
     ) = gen_generic_endpoint_tests(
         endpoint_type='content_mimetype',
@@ -319,6 +356,7 @@
         test_content_language_missing,
         test_content_language_add__drop_duplicate,
         test_content_language_add__update_in_place_duplicate,
+        test_content_language_add__duplicate_twice,
         test_content_language_get,
     ) = gen_generic_endpoint_tests(
         endpoint_type='content_language',
@@ -337,6 +375,7 @@
         # the following tests are disabled because CTAGS behave differently
         _,  # test_content_ctags_add__drop_duplicate,
         _,  # test_content_ctags_add__update_in_place_duplicate,
+        _,  # test_content_ctags_add__duplicate_twice,
         _,  # test_content_ctags_get,
     ) = gen_generic_endpoint_tests(
         endpoint_type='content_ctags',
@@ -743,6 +782,7 @@
         test_content_metadata_missing,
         test_content_metadata_add__drop_duplicate,
         test_content_metadata_add__update_in_place_duplicate,
+        test_content_metadata_add__duplicate_twice,
         test_content_metadata_get,
     ) = gen_generic_endpoint_tests(
         endpoint_type='content_metadata',
@@ -773,6 +813,7 @@
         test_revision_metadata_missing,
         test_revision_metadata_add__drop_duplicate,
         test_revision_metadata_add__update_in_place_duplicate,
+        test_revision_metadata_add__duplicate_twice,
         test_revision_metadata_get,
     ) = gen_generic_endpoint_tests(
         endpoint_type='revision_metadata',