In the iTunes preferences, it is set default to "Keep Music Folder organized", meaning it copies anything you add into iTunes into it's own structured organizational folder. You should have turned this off before importing your music into iTunes. however I too have gotten confused by this before.
I suggest that you remove all music from your computer, turn off the option in iTunes, and put it back into your iTunes folder (from where ever you had it backed up), and therefore not allowing iTunes to duplicate it all.
EDIT: The reason this function is implemented in iTunes in the first place is because if people were adding music files from their desktop, documents folder, etc. into iTunes, the music in your library would be scattered all over the place, making it hard to find the actual files if needed. With the Keep Organized option turned on, a copy of anything added is moved to the iTunes Music Folder so if you need to backup your Music or something, it's all in 1 easy to find location. After you turn this feature off to import your initial songs there, I would turn it back on, as it's pretty useful. Unless you're good at putting things in the iTunes music folder and keeping them there.
