Example symptom, when deleting a message in the webmail (which moves to Trash):
l=error m="duplicating message in old mailbox for current sessions" err="link data/accounts/mjl/msg/I/368638 data/accounts/mjl/msg/J/368640: no such file or directory" pkg=webmail
Problem introduced a few weeks ago, where moving messages starting duplicating
the message first, and the copy is erased once all references (in IMAP
sessions) to the old mailbox have been removed.