Comment 3 for bug 1813044

Can be reproduced with:
  juju deploy -n 6 ./testcharms/charm-repo/quantal/lxd-profile
  juju deploy ./testcharms/charm-repo/quantal/lxd-profile-subordinate
  juju add-relation lxd-profile lxd-profile-subordinate
  juju deploy ~/charms/ubuntu --to 0
  juju add-unit ubuntu -n 5 --to 1,2,3,4,5
  juju deploy ~/charms/ntp
  juju add-relation ntp ubuntu
  - let config settle

As "one command" upgrade the charms at once:
  juju upgrade-charm lxd-profile-subordinate --path ./testcharms/charm-repo/quantal/lxd-profile-subordinate; juju upgrade-charm lxd-profile --path ./testcharms/charm-repo/quantal/lxd-profile ; juju upgrade-charm ntp --path ~/charms/ntp ; juju upgrade-charm ubuntu --path ~/charms/ubuntu

WARNING making "testcharms/charm-repo/quantal/lxd-profile-subordinate/hooks/start" executable in charm
Added charm "local:bionic/lxd-profile-subordinate-1" to the model.
-bash: wait: pid 30 is not a child of this shell
Added charm "local:bionic/lxd-profile-1" to the model.
ERROR unable to set charm profile: upgrade charm profile already in process for machine 0, profile from "local:bionic/lxd-profile-subordinate-1"
-bash: wait: pid 30 is not a child of this shell
Added charm "local:bionic/ntp-1" to the model.
-bash: wait: pid 30 is not a child of this shell
Added charm "local:bionic/ubuntu-1" to the model.
ERROR unable to set charm profile: upgrade charm profile already in process for machine 0, profile from ""

Upgrading the ubuntu and lxd-charms separately with a minutes in-between after succeeds.