Fallback script check_existing mode ignores IPv6 addresses added to the whitelist
I added an IPv6 address to the whitelist entry for a relay that was on the hard-coded fallback list.
But since check_existing mode only checks the hard-coded list, it still showed up as a new address the next time the hard-coded list was run with check_existing mode.
We could special-case this, or just re-add relays manually if this happens (it's pretty rare), or let them drop out and be added when we next rebuild the entire list.