OK, so the problem is that your IPv6 tunnel is telling you it has 2 IPv6 nameservers, but it's just actually the same server listed twice, which NM wasn't expecting and so it does something dumb, causing the warning.
OK, so the problem is that your IPv6 tunnel is telling you it has 2 IPv6 nameservers, but it's just actually the same server listed twice, which NM wasn't expecting and so it does something dumb, causing the warning.