That link is misleading and just confuses you. :) Instead read John J. Lee's great explanation at the referenced bug report. I have tested the patch and it works as expected. Without the patch:

>>> c = SimpleCookie('foo=33;foo=34')
>>> print c
Set-Cookie: foo=34

With the patch:

>>> c = SimpleCookie('foo=33;foo=34')
>>> print c
Set-Cookie: foo=33

There should be a unit test though and something in the documentation. The keys dict should be a set instead.