BUG: Fix arrow extension array assignment by repeated key (#58616)

* Failing test_setitem_integer_array_with_repeats

* Arrow ext: fix assignment by repeated key

* Update whatsnew/v3.0.0.rst

* Update doc/source/whatsnew/v3.0.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

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---------

Co-authored-by: Matthew Roeschke <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

doc/source/whatsnew/v3.0.0.rst

@@ -466,6 +466,7 @@ Sparse
 
 ExtensionArray
 ^^^^^^^^^^^^^^
+- Bug in :meth:`.arrays.ArrowExtensionArray.__setitem__` which caused wrong behavior when using an integer array with repeated values as a key (:issue:`58530`)
 - Bug in :meth:`api.types.is_datetime64_any_dtype` where a custom :class:`ExtensionDtype` would return ``False`` for array-likes (:issue:`57055`)
 
 Styler

pandas/core/arrays/arrow/array.py

@@ -1880,7 +1880,8 @@ def __setitem__(self, key, value) -> None:
                 raise ValueError("Length of indexer and values mismatch")
             if len(indices) == 0:
                 return
-            argsort = np.argsort(indices)
+            # GH#58530 wrong item assignment by repeated key
+            _, argsort = np.unique(indices, return_index=True)
             indices = indices[argsort]
             value = value.take(argsort)
             mask = np.zeros(len(self), dtype=np.bool_)

pandas/tests/extension/base/setitem.py

@@ -202,6 +202,22 @@ def test_setitem_integer_array(self, data, idx, box_in_series):
         arr[idx] = arr[0]
         tm.assert_equal(arr, expected)
 
+    @pytest.mark.parametrize(
+        "idx",
+        [[0, 0, 1], pd.array([0, 0, 1], dtype="Int64"), np.array([0, 0, 1])],
+        ids=["list", "integer-array", "numpy-array"],
+    )
+    def test_setitem_integer_array_with_repeats(self, data, idx, box_in_series):
+        arr = data[:5].copy()
+        expected = data.take([2, 3, 2, 3, 4])
+
+        if box_in_series:
+            arr = pd.Series(arr)
+            expected = pd.Series(expected)
+
+        arr[idx] = [arr[2], arr[2], arr[3]]
+        tm.assert_equal(arr, expected)
+
     @pytest.mark.parametrize(
         "idx, box_in_series",
         [