When I add a file to my library, I rename them but if I add it again it thinks its a different file?

Before, I used to add a batch of files to my library and then rename them. Then whenever I got new music, I would put them all in the same folder where the previous batch of music was(the music I added last time) I then selected all the music in that folder, including the old ones, and added it to my library. The only music that got copied was the new music even though on my iTunes, the old music was renamed so the file name would have been different to the name on windows (iTunes must have realised this and didn't copy the old music) (I think)

I have a new laptop now and have all my old music copied into the new iTunes library (using Home Sharing) Now whenever I add new music and at the same time add old music (as I did on my old PC) it would copy the new music and the old music aswell (which I don't want because it is already in my iTunes library but just under a different name)

1) Is this because when I rename the files on iTunes, iTunes thinks it's a different file that is being added?

2) Before, why could I transfer old files and they wouldn't get copied? (because iTunes has realised that it is the same file but just under a different name)

Can I do this on my new laptop?

----------Also I cannot consolidate files fore some reason. Whenever I click it and click OK, it doesn't save. So when I go back and try and consolidate the files again, the box is unchecked (when I check the box last time and clicked OK) Has this got anything to do with it?----------

Thank you for reading this and please reply if you have any answers.

Taren Rughooputh

P.S. If you don't understand anything then reply to this regarding what you don't understand and I'll try and simplify it. : )

P.P.S. Sorry it's so long!!!