posting purchase invoice

I keep trying to post invoices but I receive an error sometimes saying purchase invoice XXXX already posted. I look for the posted invoice and there is no such thing. I know this is an internal number the system produces. How do I get past this?

So you are trying to post a purchase order.

Have you gone to the “Pay-To Vendor” to see if that Invoice# already exists as one of their Vendor Ledger Entries?

The invoice has not been posted. The invoice # that the vendor gives is not the problem. IT is the internal invoice # that Nav creates that is the problem. I could delete it but there would be a gap in the series.