How do I keep the index of the duplicate element unchanged
['a', 'b', 'b', 'c', 'c', 'd']

[[0, 'a'], [1, 'b'],  [1, 'b'], [2, 'c'], [2, 'c'], [3, 'd']]

>>> map(lambda (index, word): [index, word], enumerate([['a', 'b', 'b', 'c', 'c', 'd']])
[[0, 'a'], [1, 'b'], [2, 'b'], [3, 'c'], [4, 'c'], [5, 'd']]

input = ['a', 'b', 'b', 'c', 'c', 'd']
mapping = { v:i for (i, v) in enumerate(sorted(set(input))) }
[ [mapping[v], v] for v in input ]

[ [d.setdefault(v, len(d)), v] for d in [{}] for v in input ]

>>> import itertools
>>> seq = ['a', 'b', 'b', 'c', 'c', 'd']
>>> [[i, c] for i, (k, g) in enumerate(itertools.groupby(seq)) for c in g]
[[0, 'a'], [1, 'b'], [1, 'b'], [2, 'c'], [2, 'c'], [3, 'd']]

[
    [i, x]
    for i, (value, group) in enumerate(itertools.groupby(['a', 'b', 'b', 'c', 'c', 'd']))
    for x in group
]

a = ['a', 'b', 'b', 'c', 'c', 'd']
result = []
d = {}
n = 0
for k in a:
  if k not in d:
     d[k] = n
     n += 1
  result.append([d[k],k])

[[0, 'a'], [1, 'b'], [1, 'b'], [2, 'c'], [2, 'c'], [3, 'd'], [0, 'a']]