Fix MultiIndex.union with duplicates and differing names

doc/source/whatsnew/v3.0.0.rst

@@ -1056,6 +1056,7 @@ MultiIndex
 - :func:`MultiIndex.get_level_values` accessing a :class:`DatetimeIndex` does not carry the frequency attribute along (:issue:`58327`, :issue:`57949`)
 - Bug in :class:`DataFrame` arithmetic operations in case of unaligned MultiIndex columns (:issue:`60498`)
 - Bug in :class:`DataFrame` arithmetic operations with :class:`Series` in case of unaligned MultiIndex (:issue:`61009`)
+- Bug in :meth:`MultiIndex.union` raising when indexes have duplicates with differing names (:issue:`62059`)
 - Bug in :meth:`MultiIndex.from_tuples` causing wrong output with input of type tuples having NaN values (:issue:`60695`, :issue:`60988`)
 - Bug in :meth:`DataFrame.__setitem__` where column alignment logic would reindex the assigned value with an empty index, incorrectly setting all values to ``NaN``.(:issue:`61841`)
 - Bug in :meth:`DataFrame.reindex` and :meth:`Series.reindex` where reindexing :class:`Index` to a :class:`MultiIndex` would incorrectly set all values to ``NaN``.(:issue:`60923`)

pandas/core/indexes/multi.py

@@ -3946,7 +3946,9 @@ def _union(self, other, sort) -> MultiIndex:
         if other.has_duplicates:
             # This is only necessary if other has dupes,
             # otherwise difference is faster
-            result = super()._union(other, sort)
+            result = super(MultiIndex, self.rename(result_names))._union(
+                other.rename(result_names), sort
+            )
 
             if isinstance(result, MultiIndex):
                 return result

pandas/tests/indexes/multi/test_setops.py

@@ -687,6 +687,18 @@ def test_union_keep_ea_dtype_with_na(any_numeric_ea_dtype):
     tm.assert_index_equal(result, expected)
 
 
+def test_union_duplicates_different_names():
+    # GH#62059
+    mi1 = MultiIndex.from_tuples([(1, "a"), (2, "b")], names=["x", "y"])
+    mi2 = MultiIndex.from_tuples([(2, "b"), (3, "c"), (2, "b")])
+
+    result = mi1.union(mi2)
+    expected = MultiIndex.from_tuples(
+        [(1, "a"), (2, "b"), (2, "b"), (3, "c")], names=[None, None]
+    )
+    tm.assert_index_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "levels1, levels2, codes1, codes2, names",
     [