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The Cookie Manager behaves OK if you don't select this option. If a single thread needs to login as multiple different users, rather than using the "clear" option, just make sure that the thread logs out of the application before trying to login again. (This is what a real user would have to do)
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This \[http://markmail.org/message/bis6om3wnpqtqin5 mail message\] has details on some work-rounds.