BUG: df.getitem returning copy instead of view for unique column in dup index

doc/source/whatsnew/v1.5.0.rst

@@ -809,6 +809,7 @@ Indexing
 ^^^^^^^^
 - Bug in :meth:`loc.__getitem__` with a list of keys causing an internal inconsistency that could lead to a disconnect between ``frame.at[x, y]`` vs ``frame[y].loc[x]`` (:issue:`22372`)
 - Bug in :meth:`DataFrame.iloc` where indexing a single row on a :class:`DataFrame` with a single ExtensionDtype column gave a copy instead of a view on the underlying data (:issue:`45241`)
+- Bug in :meth:`DataFrame.__getitem__` returning copy when :class:`DataFrame` has duplicated columns even if a unique column is selected (:issue:`45316`, :issue:`41062`)
 - Bug in :meth:`Series.align` does not create :class:`MultiIndex` with union of levels when both MultiIndexes intersections are identical (:issue:`45224`)
 - Bug in setting a NA value (``None`` or ``np.nan``) into a :class:`Series` with int-based :class:`IntervalDtype` incorrectly casting to object dtype instead of a float-based :class:`IntervalDtype` (:issue:`45568`)
 - Bug in indexing setting values into an ``ExtensionDtype`` column with ``df.iloc[:, i] = values`` with ``values`` having the same dtype as ``df.iloc[:, i]`` incorrectly inserting a new array instead of setting in-place (:issue:`33457`)

pandas/core/frame.py

@@ -3612,11 +3612,18 @@ def __getitem__(self, key):
         if is_hashable(key) and not is_iterator(key):
             # is_iterator to exclude generator e.g. test_getitem_listlike
             # shortcut if the key is in columns
-            if self.columns.is_unique and key in self.columns:
-                if isinstance(self.columns, MultiIndex):
-                    return self._getitem_multilevel(key)
+            is_mi = isinstance(self.columns, MultiIndex)
+            # GH#45316 Return view if key is not duplicated
+            # Only use drop_duplicates with duplicates for performance
+            if not is_mi and (
+                self.columns.is_unique
+                and key in self.columns
+                or key in self.columns.drop_duplicates(keep=False)
+            ):
                 return self._get_item_cache(key)
 
+            elif is_mi and self.columns.is_unique and key in self.columns:
+                return self._getitem_multilevel(key)
         # Do we have a slicer (on rows)?
         indexer = convert_to_index_sliceable(self, key)
         if indexer is not None:

pandas/tests/frame/indexing/test_getitem.py

@@ -357,6 +357,21 @@ def test_getitem_empty_frame_with_boolean(self):
         df2 = df[df > 0]
         tm.assert_frame_equal(df, df2)
 
+    def test_getitem_returns_view_when_column_is_unique_in_df(self):
+        # GH#45316
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=["a", "a", "b"])
+        view = df["b"]
+        view.loc[:] = 100
+        expected = DataFrame([[1, 2, 100], [4, 5, 100]], columns=["a", "a", "b"])
+        tm.assert_frame_equal(df, expected)
+
+    def test_getitem_frozenset_unique_in_column(self):
+        # GH#41062
+        df = DataFrame([[1, 2, 3, 4]], columns=[frozenset(["KEY"]), "B", "C", "C"])
+        result = df[frozenset(["KEY"])]
+        expected = Series([1], name=frozenset(["KEY"]))
+        tm.assert_series_equal(result, expected)
+
 
 class TestGetitemSlice:
     def test_getitem_slice_float64(self, frame_or_series):

pandas/tests/indexing/test_chaining_and_caching.py

@@ -424,19 +424,6 @@ def test_detect_chained_assignment_warnings_errors(self):
             with pytest.raises(SettingWithCopyError, match=msg):
                 df.loc[0]["A"] = 111
 
-    def test_detect_chained_assignment_warnings_filter_and_dupe_cols(self):
-        # xref gh-13017.
-        with option_context("chained_assignment", "warn"):
-            df = DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, -9]], columns=["a", "a", "c"])
-
-            with tm.assert_produces_warning(SettingWithCopyWarning):
-                df.c.loc[df.c > 0] = None
-
-            expected = DataFrame(
-                [[1, 2, 3], [4, 5, 6], [7, 8, -9]], columns=["a", "a", "c"]
-            )
-            tm.assert_frame_equal(df, expected)
-
     @pytest.mark.parametrize("rhs", [3, DataFrame({0: [1, 2, 3, 4]})])
     def test_detect_chained_assignment_warning_stacklevel(self, rhs):
         # GH#42570