Comment 1 for bug 1933812

I found this on the target:

4 0 149497 1 20 0 9068 3592 - Ss ? 0:00 bash /etc/systemd/system/jujud-machine-0-exec-start.sh
4 0 149502 149497 20 0 846588 92660 - SLl ? 1:41 \_ /var/lib/juju/tools/machine-0/jujud machine --data-dir /var/lib/juju --machine-id 0 --debug

So I cleared things via:
  $ sudo /usr/sbin/remove-juju-services

That allowed me to add it again.

IMHO this is a situation that can be detected and handled much better

I'd ask to:
a) At least offer the user a better error message than "ERROR machine is already provisioned" based on the metadata you found there like
"ERROR machine is already provisioned - for controller X on IP Y, at data Z"
b) it would be very helpful to then offer "do you want to clean and re-add the machine" which would then call remove-juju-services for the user.

Right now I realize that bug 1933819 and this one come down to almost the same root cause, just once for machine-add and once for controller-boostrap. If you want to implement/fix this in one, then feel free to dup the two together.