perl -le '@arr=qw(a b a c);
my @res;
foreach(@arr) { $__=$_;$ans= grep /^$__$/,@res;
print "\$_: $_ ans: $ans";push @res,$_ unless $ans;}
 print "@res"'

$_: a ans: 0
$_: b ans: 0
$_: a ans: 1
$_: c ans: 0
a b c