BUG: duplicate indexing with embedded non-orderables (#17610)

doc/source/whatsnew/v0.22.0.txt

@@ -294,6 +294,8 @@ Indexing
 - Bug in tz-aware :class:`DatetimeIndex` where addition/subtraction with a :class:`TimedeltaIndex` or array with ``dtype='timedelta64[ns]'`` was incorrect (:issue:`17558`)
 - :func:`Index.to_series` now accepts ``index`` and ``name`` kwargs (:issue:`18699`)
 - :func:`DatetimeIndex.to_series` now accepts ``index`` and ``name`` kwargs (:issue:`18699`)
+- Bug in indexing non-scalar value from ``Series`` having non-unique ``Index`` will return value flatted (:issue:`17610`)
+
 
 I/O
 ^^^

pandas/core/series.py

@@ -664,13 +664,15 @@ def __getitem__(self, key):
             if not is_scalar(result):
                 if is_list_like(result) and not isinstance(result, Series):
 
-                    # we need to box if we have a non-unique index here
+                    # we need to box if loc of the key isn't scalar here
                     # otherwise have inline ndarray/lists
-                    if not self.index.is_unique:
-                        result = self._constructor(
-                            result, index=[key] * len(result),
-                            dtype=self.dtype).__finalize__(self)
-
+                    try:
+                        if not is_scalar(self.index.get_loc(key)):
+                            result = self._constructor(
+                                result, index=[key] * len(result),
+                                dtype=self.dtype).__finalize__(self)
+                    except KeyError:
+                        pass
             return result
         except InvalidIndexError:
             pass

pandas/tests/series/test_indexing.py

@@ -547,6 +547,26 @@ def test_getitem_setitem_periodindex(self):
         result[4:8] = ts[4:8]
         assert_series_equal(result, ts)
 
+    @pytest.mark.parametrize(
+        'result_1, duplicate_item, expected_1',
+        [
+            [
+                pd.Series({1: 12, 2: [1, 2, 2, 3]}), pd.Series({1: 313}),
+                pd.Series({1: 12, }, dtype=object),
+            ],
+            [
+                pd.Series({1: [1, 2, 3], 2: [1, 2, 2, 3]}),
+                pd.Series({1: [1, 2, 3]}), pd.Series({1: [1, 2, 3], }),
+            ],
+        ])
+    def test_getitem_with_duplicates_indices(
+            self, result_1, duplicate_item, expected_1):
+        # GH 17610
+        result = result_1.append(duplicate_item)
+        expected = expected_1.append(duplicate_item)
+        assert_series_equal(result[1], expected)
+        assert result[2] == result_1[2]
+
     def test_getitem_median_slice_bug(self):
         index = date_range('20090415', '20090519', freq='2B')
         s = Series(np.random.randn(13), index=index)