As seen in the Video when an signed email with attachments (send via KMail) is received the signature gets validated and all looks ok.
But if the email is deselected and then selected again it is revalided and after the validation message disappears the attachments are added to the email again.
So after N selection you end up with N versions of the attachments in the List.
These changes are seam to be written back to the server, as when you restart Outlook or connect from a different PC you see the emails with all those attachment copies.
Video of the bug
Email src: