TYPO3.Flow

when having, for example, a

/**
 * @param SampleDto $sampleDto {fq class name abridged}
 */
public function sampleAction(SampleDto $sampleDto);

and an appropriate Validator for that model placed in Domain\Model\Validator\SampleDtoValidator (i.e. a Domain Model Validator in the sense of [‡]), the validation for it is fired twice on action invocation.

The argument's validator stack looks like the following then:

ConjunctionValidator:
    SampleDtoValidator
    ConjunctionValidator
        GenericObjectValidator
        SampleDtoValidator

as you can see, the SampleDtoValidator is there twice, that's why it's executed twice.

If, however, the Validator is not a Domain Model Validator, means, it's not placed in Domain\Model\Validator; and annotated manually like

/**
 * @param SampleDto $sampleDto {fq class name abridged}
 * @Flow\Validate("$sampleDto", type="Acme\Foobar\Somewhere\SampleDtoValidator")
 */
public function sampleAction(SampleDto $sampleDto);

...it's validated correctly only once.
The validator stack then looks like

ConjunctionValidator:
    SampleDtoValidator
    ConjunctionValidator
        GenericObjectValidator

as you can see, the "inner" conjunction validator only has a GenericObjectValidator now.

[‡] http://docs.typo3.org/flow/TYPO3FlowDocumentation/TheDefinitiveGuide/PartIII/Validation.html#validating-domain-models