from itertools import groupby
from operator import itemgetter
my_list = [{"a": 1, "b": 2}, {"a": 1, "b": 3}, {"a": 2, "b": 4}]
it = groupby(my_list, key=itemgetter('a'))
result = sum([sum([my_dict['b'] for my_dict in group[1]]) for group in it])
print(result)