Remove from the array elements that are repeated

Question

What is the best way to remove from the array elements that are repeated. For example, from the array

a = [4, 3, 3, 1, 6, 6]

need to get

Answer 1

arr = [4, 3, 3, 1, 6, 6]

arr.
  group_by {|e| e }.
  map {|e, es| [e, es.length] }.
  reject {|e, count| count > 1 }.
  map(&:first)
# [4, 1]