That is one of the reason's my receipts are numbered sequentialy. To have a document saying they paid more the reciept number would first look different than my original duplicate if they had altered it and second if they made themselves a receipt using a similar book to mine it wouldnt' have a correct number as I would be able to show a duplicate for someone else that had whatever number they put on. That is one of the advantages of not using a computer generated system because it is easier to manipulate. Hand written in a specific book is harder to duplicate.