While restoring VM including independent disk with "delete after restore" option, existing VM(including independent disk) is deleted

Snapshot does not contain the independent disk.
Snapshot management:The vStorage Image backup method relies on being able to create virtual machine snapshots. A virtual machine snapshot is an operation that puts the virtual machine into a consistent state. All subsequent changes made to the virtual machine disks are recorded to the created snapshot. Note that snapshot operation is not supported by all virtual machine disks. For example, snapshots of independent disks are not supported; consequently, this type of virtual disks cannot be backed up using the Data Protector Virtual Environment integration. For details, see the VMware documentation.