BUG: Series.is_unique has extra output if contains objects with __ne__ defined

doc/source/whatsnew/v0.23.0.txt

@@ -1175,3 +1175,4 @@ Other
 - Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
 - Bug in accessing a :func:`pandas.get_option`, which raised ``KeyError`` rather than ``OptionError`` when looking up a non-existant option key in some cases (:issue:`19789`)
 - Bug in :func:`assert_series_equal` and :func:`assert_frame_equal` for Series or DataFrames with differing unicode data (:issue:`20503`)
+- Bug in ``Series.is_unique`` where extraneous output in stderr is shown if Series contains objects with ``__ne__`` defined (:issue:`20661`)

pandas/_libs/hashtable_class_helper.pxi.in

@@ -870,7 +870,7 @@ cdef class PyObjectHashTable(HashTable):
         for i in range(n):
             val = values[i]
             hash(val)
-            if not _checknan(val):
+            if not checknull(val):
                 k = kh_get_pymap(self.table, <PyObject*>val)
                 if k == self.table.n_buckets:
                     kh_put_pymap(self.table, <PyObject*>val, &ret)

pandas/tests/series/test_analytics.py

@@ -1594,6 +1594,22 @@ def test_is_unique(self):
         s = Series(np.arange(1000))
         assert s.is_unique
 
+    def test_is_unique_class_ne(self, capsys):
+        # GH 20661
+        class Foo(object):
+            def __init__(self, val):
+                self._value = val
+
+            def __ne__(self, other):
+                raise Exception("NEQ not supported")
+
+        li = [Foo(i) for i in range(5)]
+        s = pd.Series(li, index=[i for i in range(5)])
+        _, err = capsys.readouterr()
+        s.is_unique
+        _, err = capsys.readouterr()
+        assert len(err) == 0
+
     def test_is_monotonic(self):
 
         s = Series(np.random.randint(0, 10, size=1000))