BUG/MINOR: cfgparse: detect another uncaught case of duplicate defaults

BUG/MINOR: cfgparse: detect another uncaught case of duplicate defaults

The following sequence was not properly caught:

   defaults def
   backend back from def
   defaults def

But this one was:

   defaults def
   defaults def
   backend back from def

Let's check when defaults are declared that they're not already
referenced.

Better not backport this. While it will catch broken configs (possibly
some with backends pasted after the wrong defaults), these might still
work by accident. It may be reported as a diag warning though.