99-ipv6 is invoked twice by ifup

If the interfaces file has both inet and inet6 configured, then the 99-ipv6 script gets invoked twice and I end up with two dhclient instances being spawned.

```
auto enx000ec6a6cc6e
iface enx000ec6a6cc6e inet dhcp
iface enx000ec6a6cc6e inet6 auto
```