Dynamics GP - this document number already exists you must provide a new document number

If you have "Delete documents" unticked for a specific sales order type, GP will automatically create a record for each SOP transaction in the SOP30200 and SOP30300 tables from the moment you enter any transaction for that document type, even if you don't post it.

To resolve this, tick the "Delete documents" for the sales order type, and delete the records in the sop30200 and sop30300 tables that should not be there.