<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def warn_if_schema_already_in_spec(self, schema_key):
 """Method to warn the user if the schema has already been added to the spec. """

        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,
            )