So I'm setting a new PC. I dragged my music folders from my backup external, and put them on my new hard drive in the My Music folder.
I then installed iTunes, which, with no prompting whatsoever, automatically found the music in the My Music folder AND the identical files on my external and put everything in its library. So my iTunes library is now showing two of every song, one located on C: and one on L:, my external.
I know this should be a simple fix, but it's got me stumped. I can't find anything in the iTunes menus that will let me manually control this.