TST: add validation checks on levels keyword from pd.concat (#46654)

doc/source/whatsnew/v1.5.0.rst

@@ -92,6 +92,9 @@ Other enhancements
 - :class:`Series` and :class:`DataFrame` with ``IntegerDtype`` now supports bitwise operations (:issue:`34463`)
 - Add ``milliseconds`` field support for :class:`~pandas.DateOffset` (:issue:`43371`)
 - :meth:`DataFrame.reset_index` now accepts a ``names`` argument which renames the index names (:issue:`6878`)
+- :meth:`pd.concat` now raises when ``levels`` is given but ``keys`` is None (:issue:`46653`)
+- :meth:`pd.concat` now raises when ``levels`` contains duplicate values (:issue:`46653`)
+-
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.notable_bug_fixes:

pandas/core/reshape/concat.py

@@ -668,6 +668,8 @@ def _get_concat_axis(self) -> Index:
             return idx
 
         if self.keys is None:
+            if self.levels is not None:
+                raise ValueError("levels supported only when keys is not None")
             concat_axis = _concat_indexes(indexes)
         else:
             concat_axis = _make_concat_multiindex(
@@ -712,6 +714,10 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
         else:
             levels = [ensure_index(x) for x in levels]
 
+    for level in levels:
+        if not level.is_unique:
+            raise ValueError(f"Level values not unique: {level.tolist()}")
+
     if not all_indexes_same(indexes) or not all(level.is_unique for level in levels):
         codes_list = []
 

pandas/tests/reshape/concat/test_index.py

@@ -371,3 +371,19 @@ def test_concat_with_key_not_unique(self):
             out_b = df_b.loc[("x", 0), :]
 
         tm.assert_frame_equal(out_a, out_b)
+
+    def test_concat_with_duplicated_levels(self):
+        # keyword levels should be unique
+        df1 = DataFrame({"A": [1]}, index=["x"])
+        df2 = DataFrame({"A": [1]}, index=["y"])
+        msg = r"Level values not unique: \['x', 'y', 'y'\]"
+        with pytest.raises(ValueError, match=msg):
+            concat([df1, df2], keys=["x", "y"], levels=[["x", "y", "y"]])
+
+    @pytest.mark.parametrize("levels", [[["x", "y"]], [["x", "y", "y"]]])
+    def test_concat_with_levels_with_none_keys(self, levels):
+        df1 = DataFrame({"A": [1]}, index=["x"])
+        df2 = DataFrame({"A": [1]}, index=["y"])
+        msg = "levels supported only when keys is not None"
+        with pytest.raises(ValueError, match=msg):
+            concat([df1, df2], levels=levels)