Are you sure? Despite invoice numbers being same each invoice is attached to only one customer with only one unique destination email address for each.
Unless your customers all share the same email or are child accounts of a parent customer it would, for any ordinary software, be impossible for customer A to view customer B invoice.
So you must have a glitch, a bug that you better inform Intuit about. Of course their advice will wrongly be for you to delete your data - despite it being their problem.