When you try to modify a protection group that contains Hyper-V Virtual Machine (VM) guests or you try to remove a VM from a protection group, you receive an error message that resembles the following:
Removing Guest_Name failed:
Error 157: Guest_Name cannot be added to protection because it is already a member of a protection group.
This issue is due to duplicate entries in the DPM database. The duplicate entries may occur after a protected virtual machine is migrated from one Hyper-V cluster to a different Hyper-V cluster and then that VM is re-protected on the new cluster.