How do I add an additional IPv6 address to /etc/network/interfaces?

this question How do I add an additional IP address to /etc/network/interfaces? mostly asks what i want except that i want to add more IPv6 addresses in the same interface eth0 without incrementing to eth0.1 and so on. the ifconfig command does IPv6 like ifconfig eth0 add ... so ... how can i add more IPv6 addresses to eth0?

It would appear (tested with ifupdown version 0.7.53.1) that we can add several iface eth0 inet6 stanzas to the interfaces file, which is more declarative than the accepted answer. The following code instructs the ifupdown suite to use stateless autoconfiguration and two additional static IPv6 addresses for the eth0 network interface:

iface eth0 inet6 auto
iface eth0 inet6 static
  address 3ffe:ffff::dead:beef
  netmask 32
iface eth0 inet6 static
  address 3ffe:ffff::c0de:d00d
  netmask 32

In the question you reference, the second answer shows the equivalent solution for IPv4. In the case of IPv6, the /etc/network/interfaces file should contain something like this:

iface eth0 inet6 static
  address 2001:db8:1:2::2
  netmask 64
  # Add additional IPv6 addresses when $IFACE goes up
  up ip -6 addr add 2001:db8:1:2::3/64 dev $IFACE
  up ip -6 addr add 2001:db8:1:2::4/64 dev $IFACE
  # Remove them when $IFACE goes down
  down ip -6 addr del 2001:db8:1:2::3/64 dev $IFACE
  down ip -6 addr del 2001:db8:1:2::4/64 dev $IFACE

You will need the iproute2 package installed, but you should use ip instead of ifconfig anyway.

For adding a whole /64 to an interface: There are some Q&As in serverfault.se, like "Adding a whole IPv6 /64 block to an network interface on debian" or "Can I bind a (large) block of addresses to an interface?". Maybe they can help you.

Here is what I did for multiple v6 addresses in interfaces file. First thing to consider is there cannot be two gateways, so you add a route below the second address.

iface ens802f0 inet6 static
    address 1154::1154
    netmask 64
    gateway 1154::130


iface ens802f0 inet6 static
     address 1063::105
     netmask 64
     up ip -6 route add 1063::/64 via 1063::254 || true