iTunes help

Please don't blast me too badly, but I am all out of ideas (tried a search and apple's website)--

I purchased an external hard drive and copied my iTunes folder to it. At the same time, I copied a friend's iTunes folder to my external hard drive. My goal was to make the combined folder the directory for my iTunes music. It appears this was successful.

Long story short, iTunes now says I have 33,000 songs, but many are duplicates. When I find the duplicates in iTunes and erase either one, then try to play the other one, I get the (!) "cannot locate file" for it. I tried several experiments to isolate the problem:

1. I thought I could go to the iTunes folder and delete the duplicate files as a last resort, rather than deleting the duplicates in iTunes. However, before I delete one, if I go to the proper directory, there are NOT two files, only one. Oddly enough, when I go to the file itself in the proper directory after deleting one of the songs in iTunes, sure enough, the file is gone.

2. I tried deleting with and without the button checked that says "update iTunes Library" (or similar--I'm at work and can't recall exactly) in the edit menu (the one that supposedly makes it so that changes in iTunes are carried over to the folders themselves). It made no difference.

3. I've tried "consolidate library" in each case outlined in 2 above, doesn't fix it.

So here's where I am, by way of example: When I look at iTunes, I have "Anarchy in the UK" listed twice. When I look in my directory, there is one Sex Pistols folder, with one copy of "Anarchy in the UK" inside. If I then go to iTunes and delete either of the "Anarchy in the UK"s, regardless of whether the "update iTunes library" is checked and even if I consolidate library afterwords (or before), the remaining "Anarchy in the UK" gets a (!), and if I look at the folder in the directory, "Anarchy in the UK" is gone.

If you have experience with this issue and can help, my email address is <snipped email address>