why does my iMac has two IP addresses registered? I have been doing network ping discovery on a WiFi-LAN and one of my iMac has two IP addresses, and I cannot understand why would that be.

the only connection to the LAN is via Wifi, there is no file sharing, no Ethernet, no Bluetooth or any other network services (apart from screen sharing).
I have 4 IP addresses on some of my Mac's network interfaces since I set them up in network preferences control panel.

You could go there and delete all un-needed IP addresses or at least know it's not something that was intended.