#        1         2         3
#234567890123456789012345678901
grep/^(([@_])(?!.*\2))+$/x,<D>;

##</code><code>##

qw(a d d) 
   ad
   add
   dad

qw(a d)
   ad
   add  <= wrong... but 'dad' got correctly skipped... I think
                    add gets through because the 'd's are right
                    next to each other