Sod80

from collections import defaultdict

t = [[1, 1], [1, 2], [2, 2], [2, 4], [1, 5], [1, 6]]

d = defaultdict(int)

for item in t:
    d[item[0]] += item[1]

print(d)

defaultdict(<class 'int'>, {1: 14, 2: 6})

Process finished with exit code 0