So .bak file is a database backup of SQL Server, Duplicati backs that up, and this is the only file needed?
Still confused. The old server is dead, the new can’t restore any old .bak, so what’s being backed up now?
Are you trying to replicate your old server situation, and trying to put new .bak files onto old server backup?
As a general rule, you never want two active backups going to the same destination folder. Your situation is slightly different because I guess the old system is dead and you’re trying to continue on only the new one?
Still, it’s safer to use a new area for backups while trying to restore from the old. I hope files aren’t lost now.