restore-backup doesn't complete when trying to decrease controller numbers.

Steps to reproduce:

1) bootstrap a new controller.
2) take backup.
3) enable ha.
4) restore backup.

Expected: As stated in doc, revert back to a non ha juju.
Got:

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

See complete output down:

$ juju bootstrap --build-agent localhost ant-controller
Creating Juju controller "ant-controller" on localhost/localhost
Building local Juju agent binary version 2.1.4 for amd64
To configure your system to better support LXD containers, please see: https://github.com/lxc/lxd/blob/master/doc/production-setup.md
Launching controller instance(s) on localhost/localhost...
 - juju-a20416-0 (arch=amd64)
Fetching Juju GUI 2.9.2
Waiting for address
Attempting to connect to 192.168.0.3:22
Logging to /var/log/cloud-init-output.log on the bootstrap machine
Running apt-get update
Running apt-get upgrade
Installing curl, cpu-checker, bridge-utils, cloud-utils, tmux
Installing Juju machine agent
Starting Juju machine agent (service jujud-machine-0)
Bootstrap agent now started
Contacting Juju controller at 192.168.0.3 to verify accessibility...
Bootstrap complete, "ant-controller" controller now available.
Controller machines are in the "controller" model.
Initial model "default" added.

$ juju create-backup -m controller
20171002-062133.b56950f7-d04a-480a-8993-0b9871a20416
downloading to juju-backup-20171002-062133.tar.gz
pekkari@ant ~/workspace $ juju restore-backup -m controller --id=20171002-062133.b56950f7-d04a-480a-8993-0b9871a20416
restore from "20171002-062133.b56950f7-d04a-480a-8993-0b9871a20416" completed

$ juju enable-ha
maintaining machines: 0
adding machines: 1, 2

$ juju restore-backup -m controller --id=20171002-062133.b56950f7-d04a-480a-8993-0b9871a20416
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

Best regards.

José.