Network bridging (/etc/init.d/brmode) on the Passport sets the same IP address to the two interfaces.
During normal conditions, only eth0 is working.
If a network cable on eth0 is unplugged or if the power of the device connected on eth0 is off, then eth1 takes over with the same IP address.
There is no bridging loop and it is better to think of the two interfaces as isolated.
You can also find more details on network bridging in the Digi Passport Users Guide on page 280 located here:
Jun 06, 2019