[Flow] Problem with duplicate validation

[Sil via]

Hello,
I implemented a Validator for a model class (Country), I stored it in Validator directory and it is called automatically. The problem is that it is called twice, I followed the instructions contained in Definitive Guide "Avoid Duplicate Validation" and I declared my Validator like below:

class CountryValidator extends \TYPO3\Flow\Validation\Validator\GenericObjectValidator {
}

overriding only the isValid() method.

But still the validation is called twice. It seems that the InstanceContainier is always NULL and every time re-created.

Could you help me?

Thanks
Silvia