In section 4.5.5 we say:
One must not mark a method return value for cascaded validation more than once in a class hierarchy. In other words, sub types (be it sub classes/interfaces or interface implementations) cannot mark the return value for cascaded validation if the return value has already been marked on a super type or interface.
It would be good IMO to clarify what "class hierarchy" means, i.e. that it is ok to specify @Valid several times in parallel types.