Hello,
I have three options for you.
The first option should be safe. I strongly suggest making a back-up of your deck before you try option 2 or option 3.
Also, option 4: Duplicate cards in a other deck - #3 by dae
Option 1: tags + custom study + rename deck
- Add a distinctive tag to all the cards you want to copy; this is easily done with Anki Desktop in the card browser;
- Use “Custom Study” → “Limit to given tags” to create a custom study deck with those cards;
- Rename the Custom Study deck so it’s no longer a Custom Study deck
Caveat: the cards have not been copied. They are the exact same cards, referenced from two decks. The cards are now in two decks, and any change to a card in one deck will be reflected in the other deck.
Option 2: add-on specifically for this
https://ankiweb.net/shared/info/787914845
This add-on certainly claims to do exactly what you want. However, I haven’t tried this add-on and I have no information about how it works. It has 51 upthumbs and 26 downthumbs, which is a high number of downthumbs.
Option 3
In Anki Desktop you can select several notes in the card browser, then click menu “Notes” and “Create copy”. This works super well with only one selected note. It’s not obvious how well it works with more than one selected notes. I suggest trying on a small experimental deck first, before trying it with a confusingly high number of cards.