GH-93521: Filter out __weakref__ slot if present in bases

Lib/dataclasses.py

@@ -1156,11 +1156,16 @@ def _add_slots(cls, is_frozen, weakref_slot):
         itertools.chain.from_iterable(map(_get_slots, cls.__mro__[1:-1]))
     )
     # The slots for our class.  Remove slots from our base classes.  Add
-    # '__weakref__' if weakref_slot was given.
+    # '__weakref__' if weakref_slot was given, unless it is already present.
     cls_dict["__slots__"] = tuple(
-        itertools.chain(
-            itertools.filterfalse(inherited_slots.__contains__, field_names),
-            ("__weakref__",) if weakref_slot else ())
+        itertools.filterfalse(
+            inherited_slots.__contains__,
+            itertools.chain(
+                # gh-93521: '__weakref__' also needs to be filtered out if
+                # already present in inherited_slots
+                field_names, ('__weakref__',) if weakref_slot else ()
+            )
+        ),
     )
 
     for field_name in field_names:

Lib/test/test_dataclasses.py

@@ -3109,6 +3109,22 @@ def test_weakref_slot_make_dataclass(self):
                                     "weakref_slot is True but slots is False"):
             B = make_dataclass('B', [('a', int),], weakref_slot=True)
 
+    def test_weakref_slot_subclass_also_weakref_slot(self):
+        @dataclass(slots=True, weakref_slot=True)
+        class Base:
+            field: int
+
+        # A *can* also specify weakref_slot=True if it wants to (gh-93521)
+        @dataclass(slots=True, weakref_slot=True)
+        class A(Base):
+            ...
+
+        # __weakref__ is in the base class, not A.  But an instance of A
+        # is still weakref-able.
+        self.assertIn("__weakref__", Base.__slots__)
+        self.assertNotIn("__weakref__", A.__slots__)
+        a = A(1)
+        weakref.ref(a)
 
 class TestDescriptors(unittest.TestCase):
     def test_set_name(self):

Misc/NEWS.d/next/Library/2022-06-06-13-19-43.gh-issue-93521._vE8m9.rst

@@ -0,0 +1 @@
+Allow specifying ``weakref_slot`` several times in dataclass MRO