Your iTunes music library is a large folder and it becomes bigger and bigger with growing number of iTunes duplicates that you don't even know. My advice: check and clean up your iTunes music library on a regular basis and make sure to remove iTunes duplicates.
Here's also an Mac iTunes Duplicate Remover (officially named iTunes Music Cleanup) which simplifies the process of removing iTunes duplicates on Mac (including OS X 10.10 Yosemite and 10.9 Mountain Lion). It will find and remover duplicated items in your iTunes library.
With the best iTunes duplicate remover, it only takes you 2 steps to have all duplicate music removed. The best thing about using this software is that you don't even need to check all your music files even if you have a super big iTunes music library with thousands of songs. Just check out the quick guide below and you'll know exactly how it works. (Before you get started, download the iTunes duplicate remover to your Mac with the download button first.)
Once you run the iTunes duplicate remover, you will find that the default mode is "iTunes Music Clean". Click "Start Scan" and all music in your iTunes library will be loaded to the program.
After importing music to the program, the program will sort out duplicated songs and unidentified songs.
Click "Duplicated Songs" and you will see all the repeated items. Then choose one song in the file tray and all files of the same song will show up in the right column. Check those you want to get rid of and click "Clear Duplicate". The duplicated item will then be cleared from your iTunes library.
To make your iTunes music library clean and organized, you might also need to edit the missing album info like track, year, genre, etc. You could let the Mac iTunes Duplicate Remover give you a full guide in the post How to Identify, Edit and Fill in Music Info for iTunes Music.