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Dead weakrefs to a given object happen to be equal if they don't have a callback, but unequal if they do. However, they are always equal when alive:
>>> class O: pass
... 
>>> o = O()
>>> def cb(_): pass
... 
>>> q = weakref.ref(o)
>>> r = weakref.ref(o)
>>> s = weakref.ref(o, cb)
>>> t = weakref.ref(o, cb)
>>> q == r
True
>>> s == t
True
>>> del o
>>> q() is None
True
>>> q == r
True
>>> s == t
False
This may be related to the following optimization (?):
>>> q is r
True
>>> s is t
False

Aha, it is even worse:
>>> o = O()
>>> q = weakref.ref(o)
>>> r = weakref.ref(o)
>>> del o
>>> q() is None
True
>>> q == r
True
>>> q != r
True

New changeset 590f1b55abea by Antoine Pitrou in branch '3.2':
Issue #16453: Fix equality testing of dead weakref objects.
http://hg.python.org/cpython/rev/590f1b55abea
New changeset c00e2c1cb3a7 by Antoine Pitrou in branch '3.3':
Issue #16453: Fix equality testing of dead weakref objects.
http://hg.python.org/cpython/rev/c00e2c1cb3a7
New changeset 9e80ea48ff39 by Antoine Pitrou in branch 'default':
Issue #16453: Fix equality testing of dead weakref objects.
http://hg.python.org/cpython/rev/9e80ea48ff39 

New changeset 13b74c0b040c by Antoine Pitrou in branch '2.7':
Issue #16453: Fix equality testing of dead weakref objects.
http://hg.python.org/cpython/rev/13b74c0b040c 

Now fixed.