Master items could be the reason for that link. I think that the original sheet contains objects which linked to master items, and when you duplicate the sheet then update the object, you update the master item too.
To prevent this, on the duplicated sheet, hover your mouse on the object you want to edit, click on the Unlink icon:
Thank you for your reply, unfortunately this is not the issue. When objects are not master items they still update across the sheets. For example even if I change text (not a master item) on one sheet, the text has also been changed on the other sheet.