@11136, your question cannot be answered based on the limited information you provided. What do you mean by “I entered one invoice twice by mistake?”
Do you mean you entered two sales invoices with the same reference number? If so, you can edit one. Or you can set up automatic sequencing under Form Defaults.
Do you mean you invoiced a customer twice for the same thing? Manager will not prevent that, because customers often purchase the same thing over and over again.
Do you mean you received a sales invoice from a supplier and entered a purchase invoice twice in response? Again, Manager will not prevent that, because businesses often buy the same things over and over.
Do you mean you entered the same reference number on two purchase invoices? The solution is the same as for duplicated reference numbers on sales invoices.
In other words, what exactly is your problem? How would you expect the program to detect what happened? Why would you expect any type of notification?