MEDIUM: config: now alert when two servers have the same name

MEDIUM: config: now alert when two servers have the same name

We've been emitting warnings for over 5 years (since 1.5-dev22) about
configs accidently carrying multiple servers with the same name in the
same backend, and this starts to cause some real trouble in dynamic
environments since it's still very difficult to accurately process
a state-file and we still can't transport a server's name over the
peers protocol because of this.

It's about time to force users to fix their configs if they still
hadn't given that there is zero technical justification for doing this,
beyond the "yyp" (or copy-paste accident) when editing the config.

The message remains as clear as before, indicating the file and lines
of the conflict so that the user can easily fix it.