Fix bug in index.union with duplicates and not a subset of each other

doc/source/whatsnew/v1.3.0.rst

@@ -678,7 +678,7 @@ Interval
 Indexing
 ^^^^^^^^
 
-- Bug in :meth:`Index.union` dropping duplicate ``Index`` values when ``Index`` was not monotonic or ``sort`` was set to ``False`` (:issue:`36289`, :issue:`31326`)
+- Bug in :meth:`Index.union` dropping duplicate ``Index`` values when ``Index`` was not monotonic or ``sort`` was set to ``False`` (:issue:`36289`, :issue:`31326`, :issue:`40862`)
 - Bug in :meth:`CategoricalIndex.get_indexer` failing to raise ``InvalidIndexError`` when non-unique (:issue:`38372`)
 - Bug in inserting many new columns into a :class:`DataFrame` causing incorrect subsequent indexing behavior (:issue:`38380`)
 - Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` when setting multiple values to duplicate columns (:issue:`15695`)

pandas/core/indexes/base.py

@@ -2979,8 +2979,8 @@ def _union(self, other: Index, sort):
                 value_list.extend([x for x in rvals if x not in value_set])
                 return Index(value_list)._values  # do type inference here
 
-        elif not other.is_unique and not self.is_unique:
-            # self and other both have duplicates
+        elif not other.is_unique:
+            # other has duplicates
 
             # error: Argument 1 to "union_with_duplicates" has incompatible type
             # "Union[ExtensionArray, ndarray]"; expected "ndarray"
@@ -2989,7 +2989,7 @@ def _union(self, other: Index, sort):
             result = algos.union_with_duplicates(lvals, rvals)  # type: ignore[arg-type]
             return _maybe_try_sort(result, sort)
 
-        # Either other or self is not unique
+        # Self may have duplicates
         # find indexes of things in "other" that are not in "self"
         if self.is_unique:
             indexer = self.get_indexer(other)

pandas/tests/indexes/test_setops.py

@@ -547,7 +547,7 @@ def test_union_with_duplicate_index_and_non_monotonic(cls):
     result = a.union(b)
     tm.assert_index_equal(result, expected)
 
-    result = a.union(b)
+    result = b.union(a)
     tm.assert_index_equal(result, expected)
 
 
@@ -579,6 +579,29 @@ def test_union_nan_in_both(dup):
     tm.assert_index_equal(result, expected)
 
 
+@pytest.mark.parametrize(
+    "cls",
+    [
+        Int64Index,
+        Float64Index,
+        DatetimeIndex,
+        TimedeltaIndex,
+        lambda x: Index(x, dtype=object),
+    ],
+)
+def test_union_with_duplicate_index_not_subset_and_non_monotonic(cls):
+    # GH#36289
+    a = cls([1, 0, 2])
+    b = cls([0, 0, 1])
+    expected = cls([0, 0, 1, 2])
+
+    result = a.union(b)
+    tm.assert_index_equal(result, expected)
+
+    result = b.union(a)
+    tm.assert_index_equal(result, expected)
+
+
 class TestSetOpsUnsorted:
     # These may eventually belong in a dtype-specific test_setops, or
     #  parametrized over a more general fixture