To copy an already existing Store Item, right-click the store item you want to duplicate and copy the message ID.

Click on Copy Message ID

If you don’t see the option to copy the message ID, enable Developer Mode under Settings -> Advanced -> Developer Mode
Now use the /create type: 📦 Store Item command. Click the 💾Copy Existing button and paste the ID from the message you copied earlier

Click on Copy Exisiting

Now you will see that the entire configuration of the Store item is copied to that embed

Copied Store Item