I have a configuration with an account to read from /var/mail/username, and then move the emails to the incoming in local folders. But when deleting the messages and opening kmail again, often they are back again, duplicated and unread. Reproducible: Sometimes
I am having a similar problem. Mine started when I replaced my hard drive, reinstalled Debian/Unstable and then tried to recover my email store from the old hard drive. Not only did I not find any docs anywhere that would help me with this, The only viable/working mechanism I could find we the 'Import Kmail Archive File' choice from kmailcvt. After importing my store, then moving the imported folders, I start getting duplicate emails. I found that 'checking' the local folders account produced more duplicates. If i restart kmail, more duplicates. Then, I deleted the offending directory from my hard drive, "/home/fred/.local/share/local-mail/ebay", at which point kmail promptly reset the email store - it deleted every email and folder in kmail and presented me with an empty store with default folders, all empty. So now I am importing it all again, hoping the same problem wont recur.
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.