I have several mail filters active, which move mail upon reception from the incoming to different local folders. Sometimes, filtered/moved mails appear twice in the destination folders, and sometimes I cannot delete the doubled mails.
There are no double mails in the incoming folder, it only happens after filtering.
Maybe of importance: for some reasons, I (still) use pop3 to fetch the mails from the server.
Generally, the doubled mails fail to display with the message "Unable to fetch item from backend (collection -1): Unable to retrieve item from resource: Multiple merge candidates, aborting." This can be "fixed" by clearing the caches using the Akonadi console.
This behavior appears only with one of the recent KMail versions, maybe not before 5.5.2, however I am not totally sure.
same problem here.
KDE Frameworks 5.38.0
Qt 5.9.1 (kompiliert gegen 5.9.1)
It happens only with filtered mail - sometimes....
Fix is: clearing caches by using he Akonadi Console
I also have the same problem: moved mails sometimes appear 2-3 times in the destination folder and still a copy remains in the inbox.
Other times mails are not moved at all, at least not automatically. Doing so by hand moves them to the correct folder.
KDE Frameworks 5.44.0
Qt 5.10.1 (built against 5.10.1)
The xcb windowing system
on Manjaro Linux 17.1.6