如何计算Ruby数组中的重复元素

arr = [1, 2, 3, 2, 4, 3, 5, 6, 7, 8, 9, 9, 10, 10, 11]

duplicates = arr.group_by { |x| x }.select { |k, v| v.size > 1 }

puts duplicates.inspect

{2=>[2], 3=>[3], 9=>[9], 10=>[10]}