Goes through the list of ClientSortControls and returns a list of
        unique _ValueHandlers. Maintains order, but drops ClientSortControls
        that have already appeared to help prevent spamming.

def _get_handler_set(cls, request, fail_enum, header_proto=None):
        """Goes through the list of ClientSortControls and returns a list of
        unique _ValueHandlers. Maintains order, but drops ClientSortControls
        that have already appeared to help prevent spamming.
        """
        added = set()
        handlers = []

        for controls in request.sorting:
            control_bytes = controls.SerializeToString()
            if control_bytes not in added:
                added.add(control_bytes)
                handlers.append(
                    cls._ValueHandler(controls, fail_enum, header_proto))

        return handlers