If
you have an existing firewall and you want to add a new firewall
for HA purposes and the new firewall has an existing configuration,
it is recommended that you
Reset
the Firewall to Factory Default Settings on the new firewall.
This will ensure that the new firewall has a clean configuration.
After HA is configured, you will then sync the configuration on
the primary firewall to the newly introduced firewall with the clean
config. You will also have to configure local IP addresses.