Validation caching merges constraints for class and all of its parents. It's wrong behaviour, because the class already contains constraints from parent classes. As a result we have duplicated constraint.

The way to reproduce

Create two models, the second one extends the first one:

namespace AppBundle\Model;

class Foo
{
    public $foo;
}

namespace AppBundle\Model;

class Bar extends Foo
{
}

Create validation config, add constraint to the first class, the second class should be defined (empty or with some constraints)

AppBundle\Model\Foo:
    constraints:
        - Expression: "this.foo !== null"

AppBundle\Model\Bar:

And validation of new Bar object will generates two duplicated violations:

$container->get('validator')->validate(new Bar()));

Note: this code should be executed in prod environment, because test environment doesn't use validation cache by default.