PayPal blocking duplicate invoice id in relation to what?

Question

According to the documentation, PayPal can prevent duplicate purchases by blocking a payment when the invoice field's value has already been used in a previous purchase. My question is, used invoice here is in relation to what - my merchant account as a whole, or only for the current item_number?

Answer 1

The duplicate invoice feature is for invoices that you have generated for the life of your PayPal Account. Although you do have the ability to turn off the ability to block duplicate invoices in the Payment Receiving preferences section of your PayPal Account, it is not recommended.

Part of the reason this was created was to help protect your PayPal account from duplicate orders and payments.

Answer 2

An invoice number is generally unique across the whole business.