def test_merge_by_id_non_unique_base(self):
        schema = {
            "mergeStrategy": "arrayMergeById",
        }

        base = [
            {'id': 'a'},
            {'id': 'a'}
        ]

        head = [
            {'id': 'a',
             'foo': 1}
        ]

        merger = jsonmerge.Merger(schema)

        with self.assertRaises(BaseInstanceError) as cm:
            merger.merge(base, head)

        self.assertEqual(cm.exception.value.ref, '#/1')