Revision 339281

MFC r306729: 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.

PR:		226908, 229514
Sponsored by:	The FreeBSD Foundation