You may also see the error after the NetWorker client upgraded with the same host name which created a duplicate NSR peer information for the client.
samfsdump can back up 100,000 files a minute, because it's not backing up the full files, just control data. Meaning restore time is also VERY fast.
NetWorker uses a registry entry to keep a list of files which it will not never backup regardless of the saveset or directive that you set.
NetWorker is a product that integrates with a large number of backup storage systems and clients, with plugins for a large number of applications.