[TST]: Add test for duplicate keys in concat (pandas-dev#36556)

* Add test for 20816

* Modify test

* Move test

Author: phofl

pandas/tests/reshape/concat/test_concat.py

@@ -554,3 +554,21 @@ def test_concat_preserves_extension_int64_dtype():
     result = pd.concat([df_a, df_b], ignore_index=True)
     expected = DataFrame({"a": [-1, None], "b": [None, 1]}, dtype="Int64")
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("keys", "integrity"),
+    [
+        (["red"] * 3, True),
+        (["red"] * 3, False),
+        (["red", "blue", "red"], False),
+        (["red", "blue", "red"], True),
+    ],
+)
+def test_concat_repeated_keys(keys, integrity):
+    # GH: 20816
+    series_list = [Series({"a": 1}), Series({"b": 2}), Series({"c": 3})]
+    result = concat(series_list, keys=keys, verify_integrity=integrity)
+    tuples = list(zip(keys, ["a", "b", "c"]))
+    expected = Series([1, 2, 3], index=MultiIndex.from_tuples(tuples))
+    tm.assert_series_equal(result, expected)