How do I stop itunes from continuing to make duplicates of songs?

Last weekend I manually removed all duplicate songs in my library-from about 36 days of music down to about 24 days. It took hours! Months ago, when I had this problem, I went into preferences under advanced and changed it so it doesn’t copy files to music library when adding them. That box remians unchecked. Still last night, I realized that all the duplicates have come back, in fact I have 40 days of music. And, while I am fairly computer saavy, many of the drastic measures to fix this scare me. Should I just give in and buy the $40 software that cleans the itunes library. I can afford it, it just makes me crazy in principle that I would have to do this. Any advice?