I was the one who posted the original problem and so far I havent had a problem YET. I just know that it will be I know that if you try to print the invoice it is a problem. I still have not heard a reply.

Confirm the original invoice is truly posted by checking to see if it exists in SO-H.  Only posted invoices are available in that screen.  If you confirm that it is in fact posted, use Maintain Database on the BKARINV file to change the invoice number BEFORE posting the second invoice for the backorder.

If you do a lookup in SO-H, do you in fact have 2 entries listed for the same invoice number?

The only time we experience duplicate invoice numbers is when the first invoice is not posted (SO-G) and someone processes another sales order release (SO-E) against that same sales order.  
 
It is very important that when the first release and invoice is printed, that you answer "yes" to the prompt that says "mark these invoices as printed" and then make sure this invoice is posted (SO_G) before doing the next release.  If someone attempts to do a second release, DBA will give you a flag that there is an invoice pending against this sales order.
 
lmk223a