Validate controls list.

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

def _validate_controls(self, proposal):
        '''Validate controls list.

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