why do I have duplicate songs in Itunes?
There are many variables that could affect the outcome you have proposed. You could have downloaded physical CD's while offline years ago and then online recently, changed Apple IDs, shared an Apple ID etc etc. Theres really no telling. If the music is from the iTunes store that you downloaded it could just simply be an iCloud issue or a iTunes Match issue etc etc. I would suggest you don't go Apples route if you have a large library of duplicates because:


It only searches for duplicate 'Song Titles'
They have to be manually deleted
There is no filter option after you click 'Show Duplicates' other than 'Same Album'


However,

This is the way Apple has offered:

Go to your iTunes and in the menu bar click 'View' and in the drop down menu select 'Show Duplicates' or 'Show Duplicate Items' (based on your iTunes version)

This will generate a new table view of nothing but duplicate Song Titles. As mentioned above, it only narrows it down to that and you have to manually delete them. However, the only other filter option you have is the 'Same Album' tab which segregates only the Song Titles that repeat within the same Album Title. See picture:



If this isn't as flexible as you like, there are Apps out there can decipher it better than Apple has provided, but i will not post links to those softwares to remain objective and not sound like I am sponsoring them.