BUG: reindexing empty CategoricalIndex would fail if target had duplicates

doc/source/whatsnew/v1.3.0.rst

@@ -192,6 +192,7 @@ Categorical
 - Bug in ``CategoricalIndex.reindex`` failed when ``Index`` passed with elements all in category (:issue:`28690`)
 - Bug where constructing a :class:`Categorical` from an object-dtype array of ``date`` objects did not round-trip correctly with ``astype`` (:issue:`38552`)
 - Bug in constructing a :class:`DataFrame` from an ``ndarray`` and a :class:`CategoricalDtype` (:issue:`38857`)
+- Bug in :meth:`DataFrame.reindex` was throwing ``IndexError`` when new index contained duplicates and old index was :class:`CategoricalIndex` (:issue:`38906`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/indexes/base.py

@@ -3745,9 +3745,13 @@ def _reindex_non_unique(self, target):
                 # need to retake to have the same size as the indexer
                 indexer[~check] = -1
 
-                # reset the new indexer to account for the new size
-                new_indexer = np.arange(len(self.take(indexer)))
-                new_indexer[~check] = -1
+                if len(self):
+                    # reset the new indexer to account for the new size
+                    new_indexer = np.arange(len(self.take(indexer)))
+                    new_indexer[~check] = -1
+                else:
+                    # GH#38906
+                    new_indexer = np.arange(0)
 
         if isinstance(self, ABCMultiIndex):
             new_index = type(self).from_tuples(new_labels, names=self.names)

pandas/tests/indexing/test_categorical.py

@@ -533,3 +533,19 @@ def test_loc_with_non_string_categories(self, idx_values, ordered):
         result.loc[sl, "A"] = ["qux", "qux2"]
         expected = DataFrame({"A": ["qux", "qux2", "baz"]}, index=cat_idx)
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "cat_idx",
+        [
+            # No duplicates
+            CategoricalIndex(["A", "B"]),
+            # Duplicates: GH#38906
+            CategoricalIndex(["A", "A"]),
+        ],
+    )
+    def test_reindex_empty(self, cat_idx):
+        df = DataFrame(columns=CategoricalIndex([]), index=["K"], dtype="f8")
+
+        result = df.reindex(columns=cat_idx)
+        expected = DataFrame(index=["K"], columns=cat_idx, dtype="f8")
+        tm.assert_frame_equal(result, expected)