Validate layers list.

        Makes sure only one instance of any given layer can exist in the
        layers list.

def _validate_layers(self, proposal):
        '''Validate layers list.

        Makes sure only one instance of any given layer can exist in the
        layers list.
        '''
        self._layer_ids = [l.model_id for l in proposal.value]
        if len(set(self._layer_ids)) != len(self._layer_ids):
            raise LayerException('duplicate layer detected, only use each layer once')
        return proposal.value