dhcpcd-discuss

More generally, I'd invite you to check out how dhcpcd behaves with
eth0 and wlan0, when the same IP is assigned to both, via dnsmasq's
dhcp-host option. From the man page:

              As a special case, in DHCPv4, it is possible  to  include
             more    than    one   hardware   address.   eg:   --dhcp-
             host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.2 This
             allows an IP address to be associated with multiple hard‐
             ware addresses, and gives dnsmasq permission to abandon a
             DHCP  lease to one of the hardware addresses when another
             one asks for a lease. Beware that  this  is  a  dangerous
             thing  to  do,  it will only work reliably if only one of
             the hardware addresses is active at any time and there is
             no  way for dnsmasq to enforce this. It is, for instance,
             useful to allocate a stable IP address to a laptop  which
             has both wired and wireless interfaces.

AFAIK, this doesn't operate properly with dhcpcd.