code: def _check_field_names_unique(self, existing_item, patched_item):
          
          existing_fields_by_name = {f.name: f for f in existing_item.fields}
          for patched_field in patched_item.fields:
              if patched_field.name in existing_fields_by_name.keys():
                  existing_field = existing_fields_by_name[patched_field.name]
                  raise InvalidSpec('Patched field {} overrides pre-existing field in {} ({}:{}).'
                      .format(quote(patched_field.name),
                              quote(patched_item.name),
                              existing_field.path,
                              existing_field.lineno), patched_field.lineno, patched_field.path)
docs: Enforces that patched fields don't already exist.