Double posting - Vendor Reference field

Hello,

As you know SAP standard is checking the following fields to identify if an invoice is duplicated:

Company code

Vendor number

Currency

Reference number

Amount in document currency

Document date

I want to change the warning message that SAP shows when using the same Reference when posting (FB60) for an error message.

Which is the best way:

-Function module to check Reference field

-Add a substitution for document type (vendor invoice)

-Or another suggestion....

Thanks for your help

REgards