resolvconf(8) always leaves original DNS server in the list, allowing DNS requests to leak

[Yuri]

I am looking at this typical situation: the VPN app creates and sets up 
tap0 interface meant to be the new default route.
Then it calls this command:
 > echo "
 > nameserver <new-DNS-server>
 > domain
 > " | resolvconf -a tap0

Problem:
/etc/resolv.conf now looks like this:
 > nameserver <new-DNS-server>
 > nameserver <old-DNS-server>
The old DNS server is left at the last position. This means that in 
cases when the new server fails, DNS resolution falls back on the old 
server, therefore allowing DNS requests to leak.

I looked through the resolvconf man page, and can't find any way that 
application can replace the old DNS server there. It can only add the 
new one for some interface, and in the end remove it. The new server 
"overrides" the old one, but still leaves the old one there. This 
creates the situation when DNS leaks to the old server.

I would like to suggest the new option:
 > -x    Make the new DNS server exclusive.
With this option resolvconf(8) will replace the old server with the new one.

This will require resolvconf to have some more logic.

Yuri