Bug 10736: show duplicate invoice warning on second invoice

Bug 10736: show duplicate invoice warning on second invoice

If you have AcqWarnOnDuplicateInvoice set to warn it only warns if you have 2
or more invoices with the same number. It should warn if you're trying to
create a duplicate.

To test:
1) Turn on AcqWarnOnDuplicateInvoice.
2) Try to create an invoice that duplicates an invoice number you are already
   using exactly once.
3) Note that you get a warning after applying this patch.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
all tests pass

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>