BUG: CategoricalIndex allows reindexing with non-unique CategoricalIndex

Author: qwhelan

pandas/core/indexes/category.py

@@ -522,7 +522,7 @@ def reindex(self, target, method=None, level=None, limit=None,
 
         target = ibase.ensure_index(target)
 
-        if not is_categorical_dtype(target) and not target.is_unique:
+        if not target.is_unique:
             raise ValueError("cannot reindex with a non-unique indexer")
 
         indexer, missing = self.get_indexer_non_unique(np.array(target))

pandas/tests/indexes/test_category.py

@@ -540,6 +540,13 @@ def test_reindex_dtype(self):
         tm.assert_numpy_array_equal(indexer,
                                     np.array([0, 3, 2], dtype=np.intp))
 
+        with pytest.raises(ValueError, match='non-unique indexer'):
+            c.reindex(['a', 'a', 'c'])
+
+        with pytest.raises(ValueError, match='non-unique indexer'):
+            c.reindex(CategoricalIndex(['a', 'a', 'c'],
+                                       categories=['a', 'b', 'c', 'd']))
+
     def test_reindex_empty_index(self):
         # See GH16770
         c = CategoricalIndex([])