Sorry, forgot the "header": it's an IMAP online account

The "mails-get-doubled" effect is now just happening with those huge mails:
KMail is downloading them all again and re-uploading them, but leaving the old one also there.

This time, out of 4 mails I got 12 - each being ~7MB.

While I've prepared a screenshot of it, KMail kept downloading and re-uploading the mail with subject "bilda1" in the screenshot, where I know have 6 of. "bildas" have been two mails originally (with the same subject), but one of them has been three-plicated. "bilda2" and "bilda3" are only present once.

I've enabled the "Filter log" during this process, but it always says "0 =" for every rule, which I think means "0 matches"?

Created an attachment (id=15501) [details]
A screenshot, displaying the re-uploaded mails (but not the described end
result)

I've just had happening it again. A mail with a ~2MB attachment got downloaded
and uploaded endlessly.
Before this happend, I had about 250 duplicated small mails, which I deleted
and after KMail did not show some new mails, restarted it (and the issue with
the ~2MB mail appeared).

I had to close KMail and remove the index files with:
  find ~/.kde/share/apps/kmail/imap \( -name *.index -o -name *.index.ids \) 
-print0 | xargs -0 /bin/rm
to fix it.

So, there are actually two separate bugs in this report:
- KMail should not download and re-upload files, if there are no body type
filters
- Somehow the filtering mechanism gets into an endless loop and keeps
duplicating existing/new mails!

I'd modify your wishes like this:
- KMail should only download the whole mail if there are body-type filters
- KMail should _never_ delete the mail if no filter applies
- KMail should delete the mail only if it is filtered to a local folder but not if it's filtered to another folder on the server (is that even possible?)

The current behaviour keeps me from using KMail at home as well as at work. That's because the reuploading will cause the "other" machine to think this mail is new. So it will run all filters on it again. Thereby reuploading the mail. Now the first machine thinks it's new, and so on. 

Setup: - Courier IMAP-Server
       - KMail 1.9.5
       - Filters: only the rules for using spambayes as spam-filter

*** This bug has been confirmed by popular vote. ***

Another issue which seems to be related is that a filtered mail, which does not
get moved anywhere else (and therefor stays in the inbox) gets removed at the
current location in the message list and moved to the top of the list - at
least when ordering by "Date (Order of Arrival)".

*** Bug 121230 has been marked as a duplicate of this bug. ***

*** Bug 144988 has been marked as a duplicate of this bug. ***

Raising Severity due to duplicate bug 144988: Canceling filtering does not
re-upload the message and thus it is lost (same issue basically)

See also bug 134802.
Bug 95064 is probably also a consequence of this.

*** Bug 119553 has been marked as a duplicate of this bug. ***

Could also be the same as bug 128036.

Yes, bug 128036 is the same.

*** Bug 128036 has been marked as a duplicate of this bug. ***

*** Bug 148010 has been marked as a duplicate of this bug. ***

*** Bug 134802 has been marked as a duplicate of this bug. ***

*** Bug 145162 has been marked as a duplicate of this bug. ***

See also bug 121866, might be the same.

I really hope this gets fixed soon.

Indeed, this behaviour is highly annoying.

I think the duplication is because of the synchronization/locking problem
described in the other bug report (https://bugs.kde.org/show_bug.cgi?id=95064),
and it arises in the following condition:

slow up/download speed, automatic check for new mail in relatively short
periods:

Large message is downloaded, deleted on the server, filtered, then uploaded
again (I'm not sure when exactly the deletion takes place). Before the mail is
effectively deleted a new check starts, starting the new procedure again. The
bandwidth demand duplicates, making the problem even worse, and this continues
infinitely.

Another annoying, related behavior:

When unfiltered/uncached mails are found in an IMAP folder, they start
disappearing from the list as KMail downloads and deletes them. They become
visible again only when the whole filtering process is finished. (I think this
is a KMail for KDE4 issue only).

System: Debian unstable/experimental, KMail 4.1.2-1 (Debian versioning)

"When unfiltered/uncached mails are found in an IMAP folder, they start
disappearing from the list as KMail downloads and deletes them. They become
visible again only when the whole filtering process is finished. (I think this
is a KMail for KDE4 issue only)."

No, I've experienced that on KDE3 KMail for years.

Yep, its the case in KDE-3.5 as well. IT has prohibited the use of kmail as
messages from shared mailboxes kept disappearing.

This download-upload behavior has has caused me to lose an entire folder's
contents (it just had to be 'Archive'). KMail was happily looping and
duplicating when I finally got fed up and started pressing cancel on every
single item in in the queue pop-up. KMail crashed and the contents of the
folder vanished in the meantime.

This whole concept of download-reupload is idiotic at best. I was pleasantly
surprised at KMail + KDE4's nice interface and was 'trying it out' as a
Thunderbird replacement and added my IMAP accounts. Now I have to try and get
email backups from a shared host company. An email app with a bug this serious
should not be in a production release.

(In reply to comment #25)
I apologize for the false alarm. Turns out I did not actually lose data; I had
already moved the archived mails to a local machine for safekeeping. However, I
did experience the download/upload/duplicate loop until I removed all message
filters.

The reupload behaviour is very annoying. It seems, kmail adds one or two
headers to each Mail? Is there some way to disable this? I even do not like it
very much, if a Mailprogram alters existing Mail. Who need stuff like X-Length,
if the length could be determined at display time?

Confirm on Kmail version 1.11.2 with KDE 4.2.2.
After entering a folder of a new IMAP account with many emails, I spend a lot
of time when Kmail load the folder!
1. Kmail download all messages with body and attachments
2. After that I see that all messages removes from the list on display
one-by-one.
3. Kmail uploads all messages again to account and I see that all messages
return to list one-by-one.

I can't found how I can disable this. I remove all filters and try to disable
Anti-spam bogofilter (how can I totally disable it?) but it isn't help.

*** Bug 172111 has been marked as a duplicate of this bug. ***

*** Bug 171563 has been marked as a duplicate of this bug. ***

This issue long kept me away from using IMAP in kmail. It has been fixed later
in the 4.2 line (4.2.3?). I'm sure it is fixed because I use kmail for IMAP for
over half a year from now.

Please reopen this bug when you can reproduce this problem - from my experience
it is fixed.