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

Without introducing the need for a separate copy of the original array and using inject:

[4, 3, 3, 1, 6, 6].inject({}) {|s,v| s[v] ? s.merge({v=>s[v]+1}) : s.merge({v=>1})}.select {|k,v| k if v==1}.keys
 => [4, 1]