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

Author: gloryfromca

doc/source/whatsnew/v0.22.0.txt

@@ -183,6 +183,7 @@ Indexing
 - Bug in :class:`IntervalIndex` where empty and purely NA data was constructed inconsistently depending on the construction method (:issue:`18421`)
 - Bug in ``IntervalIndex.symmetric_difference()`` where the symmetric difference with a non-``IntervalIndex`` did not raise (:issue:`18475`)
 - Bug in indexing a datetimelike ``Index`` that raised ``ValueError`` instead of ``IndexError`` (:issue:`18386`).
+- Bug in ``Series`` containing duplicate indexing when gets embedded non-orderables or orderables, raises error or returns unexpected result. (:issue:`17610`)
 
 
 I/O

pandas/core/series.py

@@ -657,12 +657,12 @@ def __getitem__(self, key):
         try:
             result = self.index.get_value(self, key)
 
-            if not is_scalar(result):
+            if not is_scalar(result) and key in self.index:
                 if is_list_like(result) and not isinstance(result, Series):
 
                     # we need to box if we have a non-unique index here
                     # otherwise have inline ndarray/lists
-                    if not self.index.is_unique:
+                    if not is_scalar(self.index.get_loc(key)):
                         result = self._constructor(
                             result, index=[key] * len(result),
                             dtype=self.dtype).__finalize__(self)

pandas/tests/series/test_indexing.py

@@ -546,6 +546,22 @@ def test_getitem_setitem_periodindex(self):
         result[4:8] = ts[4:8]
         assert_series_equal(result, ts)
 
+    def test_getitem_with_duplicates_indices(self):
+        # GH 17610
+        s = pd.Series({1: 12, 2: [1, 2, 2, 3]})
+        s = s.append(pd.Series({1: 313}))
+        s_1 = pd.Series({1: 12, },)
+        s_1 = s_1.append(pd.Series({1: 313}))
+        assert_series_equal(s[1], s_1, check_dtype=False)
+        assert s[2] == [1, 2, 2, 3]
+
+        s = pd.Series({1: [1, 2, 3], 2: [1, 2, 2, 3]})
+        s = s.append(pd.Series({1: [1, 2, 3]}))
+        s_1 = pd.Series({1: [1, 2, 3], })
+        s_1 = s_1.append(pd.Series({1: [1, 2, 3]}))
+        assert_series_equal(s[1], s_1, check_dtype=False)
+        assert s[2] == [1, 2, 2, 3]
+
     def test_getitem_median_slice_bug(self):
         index = date_range('20090415', '20090519', freq='2B')
         s = Series(np.random.randn(13), index=index)