I've spent a considerable amount of time in an attempt to prevent iTunes from creating duplicates when you add songs that were in the iTunes music directory but not in the library. iTunes used to create duplicates in this situation, but now it seems it's much smarter. In testing, I have found that regardless of the option in iTunes that copies tracks to the directory, iTunes will not make duplicates. So this means, if the option is on and files are already in the iTunes directory, they will not be duplicated when added. It seems iTunes is slowly whittling away the errors that would create these duplicates.
It also seems that iTunes is much more careful about creating duplicates in the auto add folder. When version 9 first came out, it was more difficult to add duplicates, but you could still force it if you added songs to the automatically add to itunes folder. Now itunes will still create duplicates but not add them to the library. So you might not notice duplicates are there, but it is still much less likely you'll accidently duplicate tracks.
So, I'm trying decide on whether to keep the option that fixes the old copy files bug. I know for the most part everyone is pretty much using the newest iTunes. In a sense, you are forced too.
Anyway, I'm still working. I'll probably record a new video here in the next few days for the next version.
Ok it seems, in the off chance there is still a possibility to duplicate songs that are added. Here's what happens. Suppose and album is already in the itunes directory but not in iTunes. Suppose now I add that same album outside the itunes directory on accident programmatically or by hand. iTunes will create duplicates and reference the duplicated files. Bad.
Again it's an off chance of something happening, but it means my hard work on keeping iTunes from doing this is a good thing.