Bug 10366: Alert librarian if an invoice number is duplicated

Some vendors ship materials from the same invoice in multiple packages.
In those cases, it would be good to notify the librarian when they enter
a duplicate invoice number, so that they can continue receiving on the
previously-created invoice, rather than creating an invoice with a
duplicate number.

To test:
1) Apply patch and run database update.
2) Make sure that you have created at least one invoice on
   acqui/parcels.pl and take note of the invoice number.
3) Try to create an invoice with the same invoice number.
4) Note that without changing your configuration this works exactly
   the same as before.
5) Turn on the AcqWarnOnDuplicateInvoice system preference.
6) Try to create a new invoice with the same number as the one you
   noted earlier.
7) Make sure you get a warning about a duplicate invoice.
8) Choose to receive on the existing invoice.
9) Confirm that you are receiving on said existing invoice.
10) Start the receiving process over, and this time choose "Create new
    invoice anyway."
11) Confirm that you are now receiving on a new invoice.