diff --git a/swh/indexer/metadata_dictionary.py b/swh/indexer/metadata_dictionary.py
--- a/swh/indexer/metadata_dictionary.py
+++ b/swh/indexer/metadata_dictionary.py
@@ -109,6 +109,7 @@
             elif k in self.mapping:
                 # if there is no method, but the key is known from the
                 # crosswalk table
+                codemeta_key = self.mapping[k]
 
                 # if there is a normalization method, use it on the value
                 normalization_method = getattr(
@@ -117,12 +118,37 @@
                     v = normalization_method(v)
 
                 # set the translation metadata with the normalized value
-                translated_metadata[self.mapping[k]] = v
+                if codemeta_key in translated_metadata:
+                    translated_metadata[codemeta_key] = self.merge_values(
+                        translated_metadata[codemeta_key], v)
+                else:
+                    translated_metadata[codemeta_key] = v
         if normalize:
             return self.normalize_translation(translated_metadata)
         else:
             return translated_metadata
 
+    def merge_values(self, v1, v2):
+        """If v1 and v2 are of the form `{"@list": l1}` and `{"@list": l2}`,
+        returns `{"@list": l1 + l2}`.
+        Otherwise, make them lists (if they are not already) and concatenate
+        them."""
+        if isinstance(v1, dict) and set(v1) == {'@list'}:
+            assert isinstance(v1['@list'], list)
+            if isinstance(v2, dict) and set(v2) == {'@list'}:
+                assert isinstance(v2['@list'], list)
+                return {'@list': v1['@list'] + v2['@list']}
+            else:
+                raise ValueError('Cannot merge %r and %r' % (v1, v2))
+        else:
+            if isinstance(v2, dict) and '@list' in v2:
+                raise ValueError('Cannot merge %r and %r' % (v1, v2))
+            if not isinstance(v1, list):
+                v1 = [v1]
+            if not isinstance(v2, list):
+                v2 = [v2]
+            return v1 + v2
+
 
 class JsonMapping(DictMapping, SingleFileMapping):
     """Base class for all mappings that use a JSON file as input."""
@@ -391,14 +417,6 @@
             }
         return self.normalize_translation(metadata)
 
-    def translate_summary(self, translated_metadata, v):
-        k = self.mapping['summary']
-        translated_metadata.setdefault(k, []).append(v)
-
-    def translate_description(self, translated_metadata, v):
-        k = self.mapping['description']
-        translated_metadata.setdefault(k, []).append(v)
-
     def normalize_home_page(self, urls):
         return [{'@id': url} for url in urls]
 
@@ -468,21 +486,11 @@
                     for license in licenses
                     if isinstance(license, str)]
 
-    def translate_author(self, translated_metadata, v):
-        k = self.mapping['author']
-        translated_metadata.setdefault(k, {"@list": []})["@list"].append(v)
-
-    def translate_authors(self, translated_metadata, v):
-        k = self.mapping['authors']
-        translated_metadata.setdefault(k, {"@list": []})["@list"].extend(v)
-
-    def translate_summary(self, translated_metadata, v):
-        k = self.mapping['summary']
-        translated_metadata.setdefault(k, []).append(v)
+    def normalize_author(self, author):
+        return {"@list": [author]}
 
-    def translate_description(self, translated_metadata, v):
-        k = self.mapping['description']
-        translated_metadata.setdefault(k, []).append(v)
+    def normalize_authors(self, authors):
+        return {"@list": authors}
 
 
 def main():