How do I copy mp3s and then edit the ID3 tags of the copies without altering the tags on the originals? I made a playlist for my brother, using tracks (.mp3) that I have on my Macbook and usually listen to with iTunes. I want to put these tracks onto a USB stick to give to him. I don't have the USB stick yet but I thought I'd get the tracks ready to put onto the stick, by copying them all into a separate folder on Finder and editing the ID3 tags so that their "album" field contains the name of the playlist and their track numbers match their positions within the playlist. I thought this would be simple enough (I grew up with Windows), so just made a new folder (under "Documents") and copied them out of iTunes into here. I then realized that it's impossible to edit ID3 tags through Finder. After overcoming my initial frustration with this, I thought that instead of downloading some third-party software I'd just copy these files into iTunes and edit their ID3 tags there. I made a new playlist within iTunes, dragged the files from the folder in Documents into this playlist and edited all the ID3 tags appropriately. When I was done I deleted the mp3s out of the folder in Documents and dragged the edited tracks from iTunes into this folder to replace them. This meant that I had exactly what I wanted (the tracks with the new ID3 tags) in the folder in Documents. 
However, I then realized that something hadn't worked as I'd expected. I thought I'd duplicated the files that had initially been in my iTunes when I'd copied them into the folder in Documents, and then had added these new (but identical) files into iTunes, which I'd then edited (meaning that their ID3 tags were no longer identical to the originals). However, it turns out that somehow iTunes, presumably recognising the files to be identical, had somehow merged them? Either that or it had never actually duplicated them in the first place? I'm really not sure how it works. Either way, the end result is that the original files have disappeared from my iTunes and from my computer; now I only have the ones with the new ID3 tags. I tried to "undo" what I'd done in iTunes, but it was impossible (the editing of the ID3 tags didn't appear in the list of actions that could be undone, for some reason). 
My questions are thus: firstly, is there a way to revert the files' ID3 tags to what they were before, other than manually? (It will be really annoying to have to go and look up the album titles and track numbers for each track and edit them manually.) And secondly, what did I do wrong? How do I achieve my simple goal of copying (i.e. duplicating) a group of mp3s and editing the ID3 tags of the copies without changing the originals?
As to your second question, I would try Tagscanner (https://www.xdlab.ru/en/). You can tell it the location of the files you want to edit tags for. So there is no danger of editing the set wrong files. It also has a batch mode. Read the documentation, as with many powerful tools, it has a lot of features that can seem overwhelming to new users. It also has a preview mode for changes that allows to to see the changes you are making before committing them.