The issue is discussed on the sfPropel15Plugin Github account.
François Zaninotto closed it although it should considered as a “won’t fix” as this is really a Propel bug and not a Symfony one.
I think it is a Propel as
sfYaml.php without class existence check.
To temporally avoid this bug,
PropelYAMLParser.php must be copied to the
/lib directory of the Sf project and the require_once must be commented.
// require_once dirname(__FILE__).'/yaml/sfYaml.php';
The sfYaml class is then autoloaded by Symfony only once.