1 line Python solution, 100%

return sum(i*(i-1) for _,i in Counter(a).items())

    in c++, remember to change the result variable in the main() function

            long long result = solve(a);

a python3 solution

from collections import Counter
def solve(a):
    C = Counter(a)
    return sum(n*(n-1) for n in C.values())

For those who are trying in Java 8, you will need to change the given method to work with long and not int.

You also need to change that in the main, that is automatically generated by Hacker Rank.