Having more than one supplier invoice with the same reference is sometimes
possible. For instance, in Switzerland, the use of BVR/ESR numbers as a type of
reference permits this. Generally, this number is unique and used only for one
invoice. However, some suppliers do use the same BVR reference for several
invoices (e.g. recurring invoices).
This commit allows to disable this check if required, which is only possible
if it is extracted from 'invoice_validate'.
Closes#15891
Guewen Baconnier
55e6764747