import collections

d = """a, 1.324171
b, 0.000126
c, 1.970941
a, 1.469649
b, 0.000124
c, 0.512929
a, 1.290920
b, 0.000118
c, 0.259524
a, 0.495958
b, 0.000123
c, 0.910949
a, 1.268038
b, 0.000118
c, 1.016419
a, 1.856081
b, 0.000120
c, 1.400075
a, 1.314131
b, 0.000140"""

L = [(i[0], i[3:]) for i in d.split('\n')]
data_dict = collections.defaultdict(int)

for i, j in L:
data_dict[i] += float(j)

print(data_dict)