I have two Raspberry PIs at home and both are running home assistant on it (I am using one and my son is using the other one to test his smart appliances). I did setup duckdns addon in one of them and I did forward the port 8123 in the router. When I try to setup port forwarding for my son’s Raspberry PI for port 8123, I am getting an error message from the router “This entry already exists”. It kinda makes sense since the incoming request to external port 8123 could be coming from any source and the router gets confused as to where to route request (My Raspberry PI or my son’s Raspberry PI").
Most routers allow you to specify a destination and source port, I’m not sure about yours, but typically you configure it to say that incoming port translates to the network address port.
For example:
Incoming port 8123 forwards to IP address x.x.x.x port 8123
Incoming port 8124 forwards to IP address y.y.y.y port 8123