def _is_unique(self):
        """Is there any other relationship with same parent and child dataframes?"""
        es = self.entityset
        relationships = es.get_forward_relationships(self._child_dataframe_name)
        n = len(
            [
                r
                for r in relationships
                if r._parent_dataframe_name == self._parent_dataframe_name
            ],
        )

        assert n > 0, "This relationship is missing from the entityset"

        return n == 1
Is there any other relationship with same parent and child dataframes?
featuretools/entityset/relationship.py