Whitelist an Individual's IPv6 Range via .htaccess
With IPv6 you have to start thinking in terms of subnets, rather than individual IP addresses. A /64 subnet is allocated to a physical LAN (or VLAN) and hosts in that subnet may be assigned addresses in that subnet in a variety of ways, and may change them arbitrarily if configured to do so (e.g. privacy addresses).
It is not possible to be certain that two different IPv6 addresses in a subnet correspond to the same machine, but you can be reasonably sure that a given IPv6 /64 corresponds to a subnet and of course all of the hosts in it.