I have set up Mozilla mail filters to distribute postings to various 
mailing lists I have subscribed (e.g. wolverine-list, guinness-list) to 
my local mailboxes. The messages are marked deleted on the IMAP server 
and don't show in the INCOMING mailbox, but at the next mail check the 
same messages are re-read from the server and inserted in the local 
mailboxes as new messages. Only compacting the INCOMING mailbox will stop 
the same messages from appearing again and again.

Result:
If I leave Mozilla running with automatic mail checking on, I get dozens 
of copies of every message in the local mailbox.

Expected result:
One posting to the mailing list appears exactly once in my local mailbox.