Edit: I just read Esmi's post above, hope this provides some necessary info for your questions about how the attachment got multiple parents.
Hi otto, no I didn't manually create the new attachment posts, and I didn't know there were duplicate attachment posts. I just wasn't surprised, because I do have images used in more than one place on the site.
I use File Gallery plugin to pull in attachments from a post and re-use them in other galleries, say the gallery of a static page. I don't know how that's accomplished in the database by File Gallery. But it is often necessary to do on this site: for example something about a boat is reported in daily news, and one of the pictures goes into a "boat news" mega-gallery on the "boats" page.