why does itunes keep making duplicates

iTunes may create duplicates if the same content is repeatedly added from outside the media folder when it is set to make copies of anything that is added to the library, or is added from an external drive that hosts the media folder that was offline when iTunes was launched.

Apple's official advice on duplicates is here: Find and remove duplicate items in your iTunes library. It is a manual process and the article fails to explain some of the potential pitfalls such as lost ratings and playlist membership, or that sometimes the same file can be represented by multiple entries in the library and that deleting one and trashing the file will break any others.

Start in the Songs view. Use File > Library > Show Duplicate Items and then click Same Album to display exact duplicates as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks, however this works best when performed immediately after the dupes have been created. If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the trash.

tt2

Welcome to the Apple Support Communities!

I understand you are seeing duplicates in your iTunes library. It looks like you have already reviewed the Find and remove duplicate items in your iTunes library. Duplicates can appear if a song is on more than one album, or if you created an MP3 version of a song, or if you imported it from a CD but also purchased a digital version. Also, if you subscribe to Apple Music, you may see the version of the song in the Apple Music catalog.
However, the duplicates made it into your library, once you follow the article instructions to delete them, the next step is to make a backup of your updated library onto an external drive. Here's how to do that:
Manage and back up your iTunes media library
Take care.

Sign out of your Apple ID, then sign back in again. This will usually eliminate cloud duplicates. In some instances you may need to remove your current downloaded item and download the copy from the store, or hide the store purchase so it stops showing up.

tt2

I have had this issue for years and Apple has never seemed to address it. I have followed the instructions in Apple's Knowledgebase and I can delete the duplicate item from my library, but it continues to show duplicates - one on my computer and one available for download - exact same item (it is not an issue with an HD and SD version of the same item).

This issue happens for hundreds, if not thousands of the items in my library. If I hide an item after deleting it, I can then no longer download the item to a different device, I have to go back to my online account and unhide it before I can download it.

These are exact duplicates, not songs on different albums, not MP3 versions, not imported from a CD, not Apple Music. Exact duplicates. I have followed ALL of the steps, including backing up my updated library - in fact, I've gone as far as checking to make sure an item is still downloadable from Apple iTunes, removing both items from my library, deleting both copies from my computer, then redownloading. Sure, for a while there is no duplicate, but eventually the item will duplicate.