Geoff,

Does this happen all the time or is it an intermittent problem?

If it is intermittent, It could be caused by a user conflict when the
invoice/credit memo counter is being updated in the system file.

That is the reason that SBT will look into armast for the invoice number
existence even after it gets the next invoice number from the system file.

Another solution would be to keep a separate numbering scheme in
your order entry system and assign an invoice number to the invoice
only when it really becomes an invoice (when shipped).

HTH

Pascal