> I'll go with:
> 
> seen = {}
> 
> ARGF.each do |elem|
>   print elem if seen.include? elem
>   seen[elem] = true
> end

Thanks, this is how ruby should look like!

here is another one (i like it a bit more
functional style):

quantities = Hash.new{|h, k| h[k]=0}
ARGF.each{|l| quantities[l.chomp.to_i] += 1}
puts quantities.delete_if{|k, v| v <= 1}.keys

cheers

Simon