What Nichelle is talking about is the Accounts Payable Constants. In the Accounts Payable Constants there is an option called "Duplicate Invoice Number Edit". You can enter one of the following values -
Y - Edit for duplication. The system issues a warning message, but does NOT require action.
N - Do NOT edit the invoice number for duplication.
H - Edit the invoice number for duplication. The system issues an error message and requires you to enter a unique invoice number.
Is it correct to assume that when you talk about the system letting you enter duplicate entries of an invoice, you are talking about the same supplier and invoice number combination, correct? If not, you will need to explain in more detail what you mean by duplicate entries. This duplicate invoice number edit is at the supplier level. So supplier A can have vendor invoice number 123 and Supplier B can also send you an invoice for invoice number 123, and that is okay.
The trick is that sometimes you do get invoices from a supplier that don't really have an invoice number (think utility bills). So you have to come up with a process for those types of invoices where you would enter some kind of unique invoice number.
Bottomline, I assume you will want to set this constant to a value = 'H'. I certainly recommend testing first in a test environment before changing your production environment. Hope this helps.