prevent ng_ipv6_netif_add() and ng_ipv6_netif_add_addr() from adding duplicates as described in https://github.com/RIOT-OS/RIOT/issues/2965
prevent ng_ipv6_netif_add() and ng_ipv6_netif_add_addr() from adding duplicates as described in https://github.com/RIOT-OS/RIOT/issues/2965