AX2012 allows duplicate AP invoice numbers for the same vendor, before posting

We've discovered an issue that seems like a bug.  When submitting a Pending Vendor Invoice in AP, AX2012 (R2) will allow us to enter the same invoice number for the same vendor, as long as the first invoice is not posted yet (e.g. the first invoice is still in an approval workflow).  Once an invoice is posted, only then will AX not allow the same invoice number for that vendor.

This seems like a bug.  Shouldn't AX validate the invoice number to check for duplicates, even when there's an unposted invoice already in the Pending Vendor Invoices?

Thanks,

John