fix: deduped assume parent is a list

* this enforces if parent is not a list to be treated as a list

invenio_records_lom/utils/metadata.py

@@ -82,6 +82,10 @@ def deduped_append(self, parent_key: str, value: Any):
         """Append `value` to `self.record[key]` if not already appended."""
         self.record.setdefault(parent_key, [])
         parent = self.record[parent_key]
+
+        if not isinstance(parent, list):
+            parent = [parent]
+
         if value not in parent:
             parent.append(value)