I quit iTunes years ago. Maybe they fixed the issue but in the older versions if you copied your MP3 files over to another machine or sync’d it with another device, it would create a duplicate. Later versions they added a checkbox hidden deep in one of the menus to prevent that but it was already too late. Drove me absolutely mad. I even bought some software to try to remove duplicates but took forever, and difficult to tell (at the time) if they were 2 different recordings of the same song (e.g. a live version and a studio version from the same album).
I spent a fortune on CDs way back in the day, and then weeks burning them to MP3 only to have iTunes mess it up entirely.
Now, I just pay Spotify. Completely turned me off to iTunes.
I just have a hard drive in my car, home and a backup. Streaming doesn’t work well where I am, + I listen to a lot of live music so spotify type streaming doesn’t make sense. Quality could be an issue too.
Even today, my play counts do not sync properly. If I play songs on my iPhone, they do not always increment as plays in my music library. Drives me crazy as I obsess over play counts. Also, the album art is always disappearing from certain albums and I have to keep restoring it. Maybe I'm anal but whenever a song plays with no cover art, I'm driven to remedy that immediately.
Like you, I spent aa fortune on compact discs back in the day. At one point, I had over 2,000 of them. So the $10 a month for unlimited access to millions of songs probably saved my marriage!