ENH: reset_index on a MultiIndex with duplicate levels raises a ValueError

pandas/core/frame.py

@@ -5838,7 +5838,12 @@ class    max    type
                         level_values, lab, allow_fill=True, fill_value=lev._na_value
                     )
 
-                new_obj.insert(0, name, level_values)
+                new_obj.insert(
+                    0,
+                    name,
+                    level_values,
+                    allow_duplicates=self.flags.allows_duplicate_labels,
+                )
 
         new_obj.index = new_index
         if not inplace:

pandas/tests/frame/methods/test_reset_index.py

@@ -328,21 +328,13 @@ def test_reset_index_multiindex_nan(self):
     )
     def test_reset_index_with_datetimeindex_cols(self, name):
         # GH#5818
-        warn = None
-        if isinstance(name, Timestamp) and name.tz is not None:
-            # _deprecate_mismatched_indexing
-            warn = FutureWarning
-
         df = DataFrame(
             [[1, 2], [3, 4]],
             columns=date_range("1/1/2013", "1/2/2013"),
             index=["A", "B"],
         )
         df.index.name = name
-
-        with tm.assert_produces_warning(warn):
-            result = df.reset_index()
-
+        result = df.reset_index()
         item = name if name is not None else "index"
         columns = Index([item, datetime(2013, 1, 1), datetime(2013, 1, 2)])
         if isinstance(item, str) and item == "2012-12-31":
@@ -374,11 +366,6 @@ def test_reset_index_multiindex_columns(self):
         result = df[["B"]].rename_axis("A").reset_index()
         tm.assert_frame_equal(result, df)
 
-        # GH#16120: already existing column
-        msg = r"cannot insert \('A', ''\), already exists"
-        with pytest.raises(ValueError, match=msg):
-            df.rename_axis("A").reset_index()
-
         # GH#16164: multiindex (tuple) full key
         result = df.set_index([("A", "")]).reset_index()
         tm.assert_frame_equal(result, df)

pandas/tests/io/pytables/test_put.py

@@ -351,7 +351,7 @@ def make_index(names=None):
             columns=["a", "b"],
             index=make_index(["date", "a", "t"]),
         )
-        msg = "duplicate names/columns in the multi-index when storing as a table"
+        msg = "cannot reindex on an axis with duplicate labels"
         with pytest.raises(ValueError, match=msg):
             store.append("df", df)