Hello,
If any of the steps you’ve tried so far caused issues with your Anki collection, you can always restore from a backup created before the problems started: How to Restore from Backup
Regarding your question about keeping the old deck and the new deck completely separate in Anki, I’m afraid what you are trying to do isn’t really possible. The cards are merging because they share the same Note ID. Anki cannot have two different cards with the same ID; they are treated as the same card.
If you really want to keep them separate, you will need to use two Anki profiles. You can create a new profile, log in to the AnkiHub website, and subscribe to the MCAT deck. Then, switch to your new Anki profile, log in to the AnkiHub add-on, and click sync to download the MCAT deck.
Please note that if you ever sign in to this AnkiHub account on the profile containing the old deck, it will attempt to download the new MCAT deck and merge them again.
Unfortunately, due to Anki’s limitations, this is the only solution for this specific goal.
Let me know if you need help with any of these steps. 