Comment 131 for bug 883459

Perhaps, this helps?

I have two maildir resources. The "normal" ~/.local/share/local-mail and an additional ~/.local/share/tmp-mail, of which I don't make backups. All e. g. mailing list mails, automatically generated status updates of my homepage etc. are filtered to that place.

Practically each and every mail filtered to the tmp-mail resource appears twice in the destination folder, alway with a physically existing duplicate file with the very same content and creation date, but a completely different file name.

Could it be that filtering mails to another maildir resource is part of the problem? Perhaps, not everybody creates multiple maildirs and thus does not always experience the filtering problem?