My fix was to log out of the iTunes store (even though I had never logged into it). Repeat log in's and log out's, and still haven't had the problem of adding files to iTunes repeat itself.
Here's a longer version:
I had/have the problem of adding new songs to my library. I do not have iTunes Match. The library is on an external drive. Over 15,000 songs. On adding, the files got copied over, I can search for them in the iTunes search bar, but they don't appear in the iTunes library list, so I couldn't select them in iTunes to play. I'm not sure if this is linked, but my cloud icons in the sidebar appeared striped, as if it's sending or receiving information constantly. I had never logged into the iTunes store using iTunes 11, but my Apple ID appeared in the store header, when clicking on the store icon.
To get the cloud icons to go away, I logged out of the iTunes store completely, by clicking sign-out (even though I had never logged in with iTunes 11). The cloud icons went away, and the added files then appeared in iTunes, without me having to re-add them.
Just as a test, while logged out of the store, I deleted the files, re-added them, and they get added properly. I then repeated while logged into the store, and they appear as well. Then I quit iTunes while logged in to the store and added new files to it, and those appeared in the library as well. On successive launches of iTunes, the cloud icons go from striped to solid fairly quickly. Not sure if that's linked.