help on the best complexity

xczheng

Given a nested list [[k1, v1], [k2, v2], ..., [kn, vn]] in Python, what is the best complexity we can get to solve this:
print all the duplicate k and associated v. For example:
[[1, "a"], [2, "b"], [1, "c"]]
print:
1, "a"
1, "c"

Thanks a lot,
C