1. A method comprising: receiving a request to perform a backup operation to backup a first virtual machine; performing the backup operation, in response to the receiving the request, wherein the performing the backup operation comprises: determining a first identifier of a first virtual disk, wherein the first virtual disk is associated with the first virtual machine; determining whether the first identifier is included in a plurality of identifiers by comparing the first identifier to the plurality of identifiers, in response to a determination that the first identifier is included in the plurality of identifiers, determining that a first backup copy of the first virtual disk is already stored in a backup storage, wherein the first virtual disk is a common virtual disk, and the first virtual disk is shared by the first virtual machine and at least a second virtual machine to store common configuration data used by both the first virtual machine and the second virtual machine updating a backup catalog to include a reference to the first backup copy of the first virtual disk already stored in the backup storage instead of creating another copy of the first virtual disk in the backup storage, wherein the reference indicates a location of the first backup copy in the backup storage, and the reference associates the first virtual machine and the second virtual machine with the first virtual disk; receiving another request to restore the first virtual machine from a backup image; detecting that the first virtual disk is the common virtual disk, in response to the reference being associated with a second virtual machine; detecting that a pointer to the common virtual disk has been modified subsequent to performance of the backup operation, wherein a user has specified that restoration of the first virtual machine should not overwrite any common virtual disks; and displaying a message to the user, indicating that the pointer has been modified subsequent to the performance of the backup operation.