The music was added manually i.e. i purchased a cd, then loaded it to Itunes on my PC (Windows 10). This means that i can store the cds away in a cabinet, but still have access to all the music (I have over 3,000 cds collected over a long period of time) in one place. I am unsure that it may be an issue with Itunes as I have the app loaded on my PC and on my MAC Pro, but the content is different i.e. not the same music is loaded on the machines, no point having 2 Itunes apps with the same music. It is confusing as there is only one set of files in the file explorer, but two sets of songs on the Itunes app. No matter which one i select, they all play music, so both sets are connected to the single file, hence when i delete one song (as suggested for duplicate songs) it deletes the file from system.