Hi Tom,
As you pointed out what is bugging us is that the "future" WAL files have timestamps older than any of the past or current ones. If this is the normal behavior on the PostgreSQL side, then I believe we should focus on the barman side to understand why duplicate files are created under "errors" directory during the backup. I am sure barman handles correctly the existence of archive_status files and "future" WAL files have timestamps older than any of the past or current ones but we have done something wrong in the past which caused this issue.
I had been reading PostgreSQL documentation and WAL file internals for the last two weeks and I was stuck. Many thanks for the clarification.
Regards,
Gokhan.