Since KEA 1.9.1, it is possible to have multiple MAC addresses assigned to a single IP.
VyOS however has no provision to configure this, setting one MAC will overwrite the one already set.
My use case:
We work a lot with embedded devices. They have both a wired and a wireless interface. When installing the embedded OS, before configuration, and for troubleshooting purposes, the wired interface is used, but once configured the devices switch over to their wireless interface.
We would like the IP to remain the same when the device switches interfaces, because of firewall configuration and backend services that use client IP for device identification purposes.