# Stop the 2 services (the 2nd one probably isn't running):
sudo systemctl stop mythbackend.service
sudo systemctl stop mythtv-backend.service

# Disable then remove the duplicate service:
sudo systemctl disable mythbackend.service
sudo rm /etc/systemd/system/mythbackend.service
sudo systemctl daemon-reload

# Reenable the Mythbuntu version of the service and start it:
sudo systemctl reenable mythtv-backend.service
sudo systemctl start mythtv-backend.service

# Verify that the Mythbuntu version of the service is running:
systemctl status mythtv-backend.service

# Double check that mythbackend.service has no links here:
ls -l /etc/systemd/system/multi-user.target.wants