diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 3a22a48c997..ef5c55d6a16 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -729,11 +729,10 @@ def __delattr__(self, name):
                 # not set on the instance itself
                 return
 
-        if name in self.__dict__:
-            object.__delattr__(self, name)
-
         obj = self._mock_children.get(name, _missing)
-        if obj is _deleted:
+        if name in self.__dict__:
+            super().__delattr__(name)
+        elif obj is _deleted:
             raise AttributeError(name)
         if obj is not _missing:
             del self._mock_children[name]
diff --git a/Lib/unittest/test/testmock/testmock.py b/Lib/unittest/test/testmock/testmock.py
index 193ae9f9acb..64e2fcf61c1 100644
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -1769,6 +1769,33 @@ def test_attribute_deletion(self):
             self.assertRaises(AttributeError, getattr, mock, 'f')
 
 
+    def test_mock_does_not_raise_on_repeated_attribute_deletion(self):
+        # bpo-20239: Assigning and deleting twice an attribute raises.
+        for mock in (Mock(), MagicMock(), NonCallableMagicMock(),
+                     NonCallableMock()):
+            mock.foo = 3
+            self.assertTrue(hasattr(mock, 'foo'))
+            self.assertEqual(mock.foo, 3)
+
+            del mock.foo
+            self.assertFalse(hasattr(mock, 'foo'))
+
+            mock.foo = 4
+            self.assertTrue(hasattr(mock, 'foo'))
+            self.assertEqual(mock.foo, 4)
+
+            del mock.foo
+            self.assertFalse(hasattr(mock, 'foo'))
+
+
+    def test_mock_raises_when_deleting_nonexistent_attribute(self):
+        for mock in (Mock(), MagicMock(), NonCallableMagicMock(),
+                     NonCallableMock()):
+            del mock.foo
+            with self.assertRaises(AttributeError):
+                del mock.foo
+
+
     def test_reset_mock_does_not_raise_on_attr_deletion(self):
         # bpo-31177: reset_mock should not raise AttributeError when attributes
         # were deleted in a mock instance
diff --git a/Misc/NEWS.d/next/Library/2018-12-09-21-35-49.bpo-20239.V4mWBL.rst b/Misc/NEWS.d/next/Library/2018-12-09-21-35-49.bpo-20239.V4mWBL.rst
new file mode 100644
index 00000000000..fe9c69d234c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-12-09-21-35-49.bpo-20239.V4mWBL.rst
@@ -0,0 +1,2 @@
+Allow repeated assignment deletion of :class:`unittest.mock.Mock` attributes.
+Patch by Pablo Galindo.