BUG: fix alignment in series ops (GH14227)

Author: jorisvandenbossche

pandas/core/ops.py

@@ -622,11 +622,14 @@ def _align_method_SERIES(left, right, align_asobject=False):
 
             left, right = left.align(right, copy=False)
 
-            index, lidx, ridx = left.index.join(right.index, how='outer',
-                                                return_indexers=True)
-            # if DatetimeIndex have different tz, convert to UTC
-            left.index = index
-            right.index = index
+            if is_datetime64tz_dtype(left.index):
+                if left.index.tz != right.index.tz:
+                    # if DatetimeIndex have different tz, convert to UTC
+                    index, lidx, ridx = left.index.join(
+                        right.index, how='outer', return_indexers=True)
+
+                    left.index = index
+                    right.index = index
 
     return left, right
 

pandas/tests/series/test_operators.py

@@ -1810,3 +1810,11 @@ def test_dti_tz_convert_to_utc(self):
 
         res = Series([1, 2], index=idx1) + Series([1, 1], index=idx2)
         assert_series_equal(res, Series([np.nan, 3, np.nan], index=base))
+
+    def test_op_duplicate_index(self):
+        # GH14227
+        s1 = Series([1, 2], index=[1, 1])
+        s2 = Series([10, 10], index=[1, 2])
+        result = s1 + s2
+        expected = pd.Series([11, 12, np.nan], index=[1, 1, 2])
+        assert_series_equal(result, expected)