TST: add test for DataFrame with duplicate indices concat

pandas/tests/frame/methods/test_join.py

@@ -3,6 +3,8 @@
 import numpy as np
 import pytest
 
+from pandas.errors import InvalidIndexError
+
 import pandas as pd
 from pandas import (
     DataFrame,
@@ -369,3 +371,11 @@ def test_frame_join_tzaware(self):
 
         tm.assert_index_equal(result.index, expected)
         assert result.index.tz.zone == "US/Central"
+
+    def test_duplicate_indices_concat_raise(self):
+        # https://github.com/pandas-dev/pandas/issues/36263
+        df1 = DataFrame(np.random.randn(5), index=[0, 1, 2, 3, 3], columns=["a"])
+        df2 = DataFrame(np.random.randn(5), index=[0, 1, 2, 2, 4], columns=["b"])
+        msg = "Reindexing only valid with uniquely valued Index objects"
+        with pytest.raises(InvalidIndexError, match=msg):
+            pd.concat([df1, df2], axis=1)