>>91　Perl5

use feature say;
use List::Util min;
for ([1,1,1,1,2,2,2,3,3,4], [1,2,3,4,5,5,4,3,2,1], [3,1,4,1,5,9,2,6,5,3,5]) {
　my %h;
　$h{$_}++ for @$_;
　@s = grep{1 == $h{$_}} keys %h;
　$" = ',';
　say "@$_ -> ", @s ? min @s : -1;
}

実行例
~ $ perl 15_91.pl
1,1,1,1,2,2,2,3,3,4 -> 4
1,2,3,4,5,5,4,3,2,1 -> -1
3,1,4,1,5,9,2,6,5,3,5 -> 2