It seems that a previous local provider instance did not get destroyed properly.
You can destroy the current one with:
# If using juju 1.17.x
juju destroy-environment -y local
# If using juju 1.16.x
juju destroy-environment -y -e local
And then look at the current lxc images:
sudo lxc-ls --fancy
Any that start with "onrea-local" will be ones that juju things it created.
You can remove them with:
sudo lxc-destroy -n onrea-local-machine-1
And so on.
This is a limitation of the local provider, and one what we are looking to fix very soon (the clean destruction that is).