You need to use np.add.at:
A = np.array([1,2,3])
B = np.array([10,20,30])
I = np.array([0,1,1])
np.add.at(A, I, B)
print(A)
prints
array([11, 52, 3])
This is noted in the doc:
ufunc.at(a, indices, b=None)
Performs unbuffered in place operation on operand ‘a’ for elements specified by ‘indices’. For addition ufunc, this method is equivalent to a[indices] += b, except that results are accumulated for elements that are indexed more than once. For example, a[[0,0]] += 1 will only increment the first element once because of buffering, whereas add.at(a, [0,0], 1) will increment the first element twice.
