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.
If I leave Mozilla running with automatic mail checking on, I get dozens
of copies of every message in the local mailbox.
One posting to the mailing list appears exactly once in my local mailbox.
I've heard reports of similar problems that were fixed long ago. 0.9.1 is in
rawhide now. It should fix problems like that.