Remove elements with even instances from a list

Question

Suppose I have a list,

  {12 -> -12, 13 -> -13, 14 -> -14, 15 -> -15, 9 -> -9, 11 -> -11, 
  13 -> -13, 15 -> -15, 5 -> -5, 7 -> -7, 13 -> -13, 15 -> -15, 
  3 -> -3, 7 -> -7, 11 -> -11, 15 -> -15}

I want to remove elements which occur even times completely and keep a single instance of elements occurring odd times. That is, I want to end up with the list,

  {12 -> -12, 13 -> -13, 14 -> -14, 9 -> -9, 5 -> -5, 3 -> -3}

Answer 1

You could use

Keys[Select[Counts[list],OddQ]]

where list is your list.