Invoice Number uniqueness validation across OU's.

Hi  Currently if there is a same invoice number coming twice(with in the same OU) ,say #1234, from the same vendor, then the system checks for the uniqueness of the invoice and rejects/warns that number. 

However, say if an invoice #1234 ,coming from a supplier is entered in an OU "US-A"  and if the same invoice number is coming from the same supplier is entered in an OU "US-B", then our business wants the system to warn that there is an already existing invoice with the same number from the same vendor. 

Basically a same invoice can be shared between two different OU's.