You can groupby
on enumerate(a)
:
>>> from itertools import groupby
>>> from operator import itemgetter
>>> indices = ((x[0] for x in g) for _, g in groupby(enumerate(a), key=itemgetter(1)))
>>> [[b[x] for x in lst] for lst in indices]
[[1, 2, 1], [3, 2], [1], [0]]