makeman: avoid bogus output with duplicated options

On some targets 'make showconfig' currently reports both 'no' and 'yes'
for some options. For example:

% make TARGET=mips showconfig | grep SSP
MK_SSP = no
MK_SSP = yes

Emit a warning on encountering a duplicated variable, and skip the
second entry.

Sponsored by: The FreeBSD Foundation