can't restore-backup more than once

[Heather Lanigan]

Any 2nd attempt at restore fails, with the same id or a different id.

$ juju version
2.3.1-artful-amd64

$ juju restore-backup --id 20180103-011436.e399be90-d5f1-4312-823f-57fdb94fe9e1 -m controller
restore from "20180103-011436.e399be90-d5f1-4312-823f-57fdb94fe9e1" completed
$ juju restore-backup --id 20180103-011436.e399be90-d5f1-4312-823f-57fdb94fe9e1 -m controller
ERROR could not clean up after failed restore attempt: cannot complete restore: <nil>: Restore did not finish succesfuly
ERROR cannot perform restore: <nil>: restore failed: error restoring state from backup: setting special user permission in db: Role "oploger@admin" already exists

$ juju create-backup -m controller
20180103-011729.e399be90-d5f1-4312-823f-57fdb94fe9e1
downloading to juju-backup-20180103-011729.tar.gz
$ juju restore-backup -m controller --id 20180103-011729.e399be90-d5f1-4312-823f-57fdb94fe9e1
ERROR could not clean up after failed restore attempt: cannot complete restore: <nil>: Restore did not finish succesfuly
ERROR cannot perform restore: <nil>: restore failed: error restoring state from backup: setting special user permission in db: Role "oploger@admin" already exists

[Heather Lanigan (hmlanigan)]

https://github.com/juju/juju/pull/8263