Duplicate List Index

list_element_index

## duplicate_list_index: given a list, return a dictionary, with the 'key' being the list
#                        elements (unique, not duplicated), and the corresponding dictionary
#                        'value' being a list containing the index location of each instance.
#
#  for example:
#    list_to_check = list('ABRACADABR')
#
#  then, this method would return the following dictionary:
#    {'A': [0, 3, 5, 7], 'R': [2, 9], 'B': [1, 8], 'C': [4], 'D': [6]}
def duplicate_list_index(list_to_check):
  # store each element instance into dictionary
  dict_duplicates = collections.defaultdict(list)
  for key, value in enumerate(list_to_check):
    dict_duplicates[value].append(key)

  # remove non-duplicates from dictionary
  for key, value in dict_duplicates.items():
    if ( len(value) <= 1 ):
      del dict_duplicates[key]
  return dict_duplicates