你可以这样做:

a_list=[[1,15],[2,17]]
b_list=[[1,1],[3,4],[5,9]]

for a in a_list:
    found = False
    for b in b_list:
        if a[0] == b[0]:
            b[1] += 1
            found = True
    if not found:
        b_list.append([a[0],1])

print(a_list)
print(b_list)

输出:

[[1, 15], [2, 17]]
[[1, 2], [3, 4], [5, 9], [2, 1]]