I migrated to kmail2. Now i have a serious problem of email duplication.

I set up my filters to move pop mails to the correct folder. First thing i saw is that the filters are not actually applied, but i have to manually apply "all filters" on the incoming emails.

Then the duplication starts. The emails are moved correctly and disapear from my inbox and the mail counter moves up in the folders mentioned. But suddenly, the emails reapear in the inbox out of nowhere. i dont know if kmail redownloads them or does something else.

This is critical, is there any advice on how to make filters not duplicate mails in kmail2?

The problem that filters where not applied at all is gone, i had to restart the computer (feels like windows?).

But the duplication problem is still here. Is there a way to delete all duplicated emails?

Now i'm also hit by https://bugs.kde.org/283819
The error message is repeated 10+ times, and i have no idea what it wants to tell me. Is there a way to get detailed error logs that could indicate what the problem is?

There should be a menu option to delete duplicate emails, under the "Folder" menu. As for the maildir warning, check that the Local Folders resource, or whatever folder resource (local) you have configured points to an existing directory.

I found a workaround for the mail duplication with the filters.

What happend is, i imported the kmail1 folder structure into kmail2. I got all my folders etc. But when a filter moved a mail into one of those folders, something must have gone wrong, because the mail was moved AND it reapeared in the inbox.
What i did now is create a new folder with every of the old folders, move all mails in them into the new folders, delete the old ones, rename the new one and modify all filters. Now they work. This is clearly a bug, apparently the interface of kmail sees more than the backend.

Then thank you for mentioning the "delete duplicate emails" tool. Sadly, it does not work. It does nothing, no error message, no progress dialog nothing. The local resource points to an existing folder, in the hidden .local folder, it is empty contains 3 subfolders, cur, new and tmp, which are empty too. No hidden files in any of those folders. I'm not really sure what that local folder resource does, since it contains nothing.

I also just saw that folders are missing, i don't have a "trash" folder in kmail for example. So this is getting really important. how can i fix kmail? something seems wrong with my folder structure, i could partially fix it, but kmail keeps complaining with the mentioned error message (+ the missing folders)