dups.rb

array = Array.new(100) { rand(1...5000) }

def dp(array)
  duplicates = {}

  array.each do |a|
    if duplicates[a]
      duplicates[a] += 1
    else
      duplicates[a] = 1
    end
  end

  duplicates.sort_by { |k,v| v }.reverse
end