Cloudstack allows you to create snapshots with the same name for the same volume

### problem

Bug report:
CloudStack allows creating multiple snapshots of the same volume with identical names.

These snapshots are treated as separate entities in CloudStack, but they reference the same snapshot file on secondary storage.

The snapshot file on secondary storage is overwritten when a snapshot with the same name is created

When one of these snapshots is deleted via CloudStack. The snapshot file is removed from secondary storage. Other snapshot entries remain in CloudStack but no longer have a corresponding file.

Deleted a snapshot in Cloudstack

The first snapshot was deleted from CloudStack. During the deletion process, CloudStack also removed the snapshot file from secondary storage.

However, a second snapshot entry still remains in the database and references a file on secondary storage that no longer exists.

### versions

ACS: 4.20.2.0
Hypervisor: KVM
Primary Storage: Linstor
Secondary Storage: NFS

### The steps to reproduce the bug

1. Create a volume
2. Create snapshot with name for example "test-snapshot"
3. Create another snapshot with the same name "test-snapshot"
4. Verify that both snapshots exist in CloudStack and only one exists on the secondary storage
5. Delete ONE of the snapshots via CloudStack
6. Check secondary storage — snapshot file is removed
7. Check remaining snapshot in CloudStack

### What to do about it?

Prevent creating snapshots with the same name for the same volume. Deleting one breaks the other.

Start by reproducing the listed steps for two snapshots with the same name on one volume, then trace the snapshot creation and deletion paths involved. Done means CloudStack rejects the duplicate, or otherwise keeps snapshot entries and secondary-storage files consistent when one snapshot is deleted.

Written by the indexing model from the issue text.