@_1 = (1,2,5,9,11,12,13);
@_2 = (1,2,5,11,12,13);
@_3 = (1,2,5,7,9,12,13);

map{ $u{$_}++ }@_1,@_2,@_3;
map{ $u{$_} != 2 and delete $u{$_} }keys %u;

print+(join ',', keys %u), $/