discover/network: Ignore interfaces with pre-existing MAC address

Petitboot uses the MAC address of network interfaces as a unique
identifier. This can cause a crash in pb-discover on a machine that has
multiple interfaces with the same MAC address.
While duplicate MAC addresses are rare and imply an issue with the
larger system configuration Petitboot should handle this gracefully, so
log a warning and ignore any interfaces other than the first to appear
that share a MAC address.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>