[Eldar9x]

Что-то вроде этого:

    class PurchFormLetter_Invoice_Ex
        extends PurchFormLetter_Invoice
    {
        boolean checkInvoice()
        {
            if (VendParameters::find().checkInvoice == ReuseVoucher::WarnDuplicate)
                return true;
            return super();
        }

        static PurchFormLetter construct(DocumentStatus     document,
                                             boolean            getParmId = true)
        {
            if (document ==  DocumentStatus::Invoice)
                return new PurchFormLetter_Invoice_Ex(getParmId);            
            return PurchFormLetter::construct(document, getParmId);
        }
    }