Hi,
There is no warning message appearing for duplicate purchase invoice. It so happens that a certain invoice may be issued by the supplier with the same invoice number and amount at different time intervals. It is very human that mistake can occur and invoice may be entered twice. There should be a warning message appearing on screen.
This feature is implemented in most softwares and helps prevent duplication especially in case of purchase invoices. I am not referring to sales invoice since it is a running series automatically generated.