>I replaced append with insert into command but this doesnt help.
Some times a day, a duplicate invoice headers will randomly
appended to file.

Have you looked at the table with no index to see if the record is duplicated, or if the index is corrupted?

The insert command should fix the inherent ambiguity with append blank. I'm still betting on a network client issue. It's amazing what a few tweaks will do. (Novell Client for Netware - adjust "Lock Retries" from 5 to 1, and we've seen 30-50% improvement in speed for our legacy apps)