Comment 1 for bug 1745671

I'm bringing this upstream ...

http://cloudinit.readthedocs.io/en/latest/topics/network-config-format-eni.html#network-config-eni

states that:
~~~
Please reference existing documentation for the /etc/network/interfaces(5) format.
~~~

From https://manpages.debian.org/jessie/ifupdown/interfaces.5.en.html - the man page doesn't seem to say anything else about duplicate iface declarations with different families:
~~~
Stanzas defining logical interfaces start with a line consisting of the word "iface" followed by the name of the logical interface. In simple configurations without mapping stanzas this name should simply be the name of the physical interface to which it is to be applied. (The default mapping script is, in effect, the echo command.) The interface name is followed by the name of the address family that the interface uses. This will be "inet" for TCP/IP networking, but there is also some support for IPX networking ("ipx"), and IPv6 networking ("inet6"). Following that is the name of the method used to configure the interface.
(...)
~~~

According to https://wiki.debian.org/NetworkConfiguration , this should work, though:
~~~
If you're configuring it manually then something like this will set the default gateway (network, broadcast and gateway are optional):

    auto eth0
    iface eth0 inet static
        address 192.0.2.7
        netmask 255.255.255.0
        gateway 192.0.2.254

If you want to add an IPv6 address, too, append something like:

    iface eth0 inet6 static
        address 2001:db8::c0ca:1eaf
        netmask 64
        gateway 2001:db8::1ead:ed:beef
~~~