I also just discovered that when attaching multiple files, sometimes adding the text for the "final save" will trigger the bug. Then, when going back, the files will be a mess with duplicates and some of them unchecked. I check all of them, save WITHOUT TEXT, and only a few are still unchecked. Check those, save WITHOUT TEXT, and now all of the attachments are checked. Add text, save, and it works.
There is obvious something wrong in both the handling of the file attachments, and some bizarre interaction between the attachments and the post text.