BUG: loc setitem with duplicate columns and new columns corrupts Data…

doc/source/whatsnew/v3.1.0.rst

@@ -267,6 +267,9 @@ Indexing
 - Bug in :meth:`Index.get_level_values` mishandling boolean, NA-like (``np.nan``, ``pd.NA``, ``pd.NaT``) and integer index names (:issue:`62169`)
 - Bug in :meth:`Index.get_loc` raising ``KeyError`` when looking up a tuple in an object-dtype :class:`Index` with duplicates (:issue:`37800`)
 - Bug in :meth:`Index.insert` silently casting booleans to numeric when used with nullable numeric dtypes like ``Float64`` or ``Int64`` (:issue:`61709`)
+- Fixed bug in :meth:`DataFrame.loc` where assigning with duplicate column names and new columns corrupted unrelated columns (:issue:`58317`)
+-
+
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -913,8 +913,7 @@ def _ensure_listlike_indexer(self, key, axis=None) -> None:
                 #  below would create float64 columns in this example, which
                 #  would successfully hold 7, so we would end up with the wrong
                 #  dtype.
-                indexer = np.arange(len(keys), dtype=np.intp)
-                indexer[len(self.obj.columns) :] = -1
+                indexer = self.obj.columns.get_indexer(keys)
                 new_mgr = self.obj._mgr.reindex_indexer(
                     keys, indexer=indexer, axis=0, only_slice=True, use_na_proxy=True
                 )

pandas/tests/indexing/test_loc.py

@@ -2398,6 +2398,28 @@ def test_loc_setitem_with_expansion_new_row_and_new_columns(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    def test_loc_setitem_with_expansion_duplicate_columns(self):
+        # GH#58317
+        df = DataFrame(
+            [[1, 2, 3, 4], [4, 5, 6, 7], [7, 8, 9, 10]],
+            columns=["D", "B", "C", "A"],
+        )
+        item = DataFrame(
+            [[1, 2, 3, 4], [4, 5, 6, 7], [7, 8, 9, 10]],
+            columns=["A", "B", "C", "X"],
+            index=[3, 2, 1],
+        )
+        df.loc[[True, False, True], ["B", "E", "B"]] = item
+        expected = DataFrame(
+            [
+                [1, np.nan, np.nan, 3, 4, np.nan],
+                [4, 5.0, 5.0, 6, 7, np.nan],
+                [7, 5.0, 5.0, 9, 10, np.nan],
+            ],
+            columns=["D", "B", "B", "C", "A", "E"],
+        )
+        tm.assert_frame_equal(df, expected)
+
 
 class TestLocCallable:
     def test_frame_loc_getitem_callable(self):