code: 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.valuf _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
docs: Validate layers list.
  
          Makes sure only one instance of any given layer can exist in the
          layers list.