> ? ? ? ?run_benchmark(bx, 'jegII', iterations) { seen =
> Hash.new(0);a.select { |e| (seen[e] += 1) > 1 }.uniq }

I think you could improve this one a bit by simply doing
{ seen = Hash.new(0); a.select { |e| (seen[e] += 1) == 2 } }

Regards
Sylvain