code: def warn_if_schema_already_in_spec(self, schema_key):
          
          if schema_key in self.openapi.refs:
              warnings.warn(
                  "{} has already been added to the spec. Adding it twice may "
                  "cause references to not resolve properly.".format(schema_key[0]),
                  UserWarning,
              )
docs: Method to warn the user if the schema has already been added to the
          spec.