TST: setitem in presence of duplicate cols (#38241)

Author: ivanovmg

pandas/tests/frame/indexing/test_indexing.py

@@ -241,6 +241,24 @@ def inc(x):
         expected = DataFrame([[-1, inc], [inc, -1]])
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize(
+        "cols, values, expected",
+        [
+            (["C", "D", "D", "a"], [1, 2, 3, 4], 4),  # with duplicates
+            (["D", "C", "D", "a"], [1, 2, 3, 4], 4),  # mixed order
+            (["C", "B", "B", "a"], [1, 2, 3, 4], 4),  # other duplicate cols
+            (["C", "B", "a"], [1, 2, 3], 3),  # no duplicates
+            (["B", "C", "a"], [3, 2, 1], 1),  # alphabetical order
+            (["C", "a", "B"], [3, 2, 1], 2),  # in the middle
+        ],
+    )
+    def test_setitem_same_column(self, cols, values, expected):
+        # GH 23239
+        df = DataFrame([values], columns=cols)
+        df["a"] = df["a"]
+        result = df["a"].values[0]
+        assert result == expected
+
     def test_getitem_boolean(
         self, float_string_frame, mixed_float_frame, mixed_int_frame, datetime_frame
     ):