As reported in 2 PRs (PR176671 and PR184149) it can happen that epair devices can have the same MAC address.
My proposal, following what hrs@ propose in PR 184149 is to use the same if_index for both interfaces.
Moreover, I add some bits of the hostid to mitigate the problem reported by Olivier in PR176671, adopting the same approach used by if_bridge(4), as suggested by Olivier itself.
Following this approach, all element of the MAC address are used to improve uniqueness.