With physical tape devices it is typical to make use of concurrency. This setting defines the number of disk agents writing to the media agent in parallel and can be found in the Advanced options for the device. As a Virtual Tape Library (VTL) is just seen as any other (physical) tape library by Data Protector, the default settings for a VTL, including the concurrency on the devices, are just the same. That means that a default concurrency of "4" will be seen on virtual LTO devices.
This is not recommended for deduplicating devices (e.g. VTL devices on StoreOnce). This is because the interleaving of data from multiple sources will not be consistent between different backups and will significantly reduce the deduplication efficiency and the performance.
So it is highly recommended to change the default concurrency of "4" to a "1" in the Advanced Options for each of the VTL devices on a deduplication hardware device.