Entries duplicated as pages

I built a site for a client (currently on 1.8.4), and when they add entries, they sometimes get duplicated as child pages. I haven't been able to duplicate it, but I set _enable_add_child_page to false in settings.yaml, so I've ruled out the possibility of the client entering content in the wrong location.

Here's what things look like when it happens:

Products            (page)
  Some Product      (entry)
  Another Product   (entry)
  Another Product   (folder)
    another-product (page)

I can't figure out a pattern either. The only configuration I'm using that I haven't on any other builds is _content_type: html.

I'm unable to delete the duplicated content via the CP. I have to use FTP to delete the files.