title: List indexes of duplicate values in a list with Python
body: <p>I am trying to modify this definition that lists duplicate items so that it lists indexes of duplicate values. Also, I would like it to list ALL of the duplicates that means the resultant for a = [1,2,3,2,1,5,6,5,5,5] would be duplicate_indexes = [3,4,7,8,9] Here's the definition:</p>
  
  <pre><code>def list_duplicates(seq):
      seen = set()
      seen_add = seen.add
      # adds all elements it doesn't know yet to seen and all other to seen_twice
      seen_twice = set( x for x in seq if x in seen or seen_add(x) )
      # turn the set into a list (as requested)
      return list( seen_twice )
  
  a = [1,2,3,2,1,5,6,5,5,5]
  list_duplicates(a) # yields [1, 2, 5]
  </code></pre>