When an External Document No. is entered on a Sales Order, verify that this number has not been used by checking the External Document No. from Customer Ledger Entries for the particular customer. If a duplicate is found, display a warning message “External Document No. XXX already exists for this customer”, where XXX is the duplicate External Document No.

This is what's currently happening for Purchase Orders and the Vendor Invoice No. where the Ext. Doc. No. on the Purchase & Payables Setup is enabled. We need the same validation on the sales side.