William James wrote:
> Eric Hodel wrote:
>
> > seen = {}
> >
> > ARGF.each do |elem|
> >    print elem if seen.include? elem
> >    seen[elem] = true
> > end
>
> seen = {}
> while s = gets
>   print s if seen.key? s
>   seen[ s ] = nil
> end
>
> Or:
>
> seen = Hash.new(0)
> while s = gets
>   print s if ( seen[s] += 1 ) > 1
> end

seen, s = Hash.new(0)
print s if ( seen[s] += 1 ) > 1 while s = gets