def pairs(n):
    return n * (n - 1) / 2

def solution(lst):
    res = 0
    stack = lst[0:1]
    lst = sorted(lst[1:])
    while(len(lst)):
        nxt = lst.pop()
        if len(stack) and stack[0] == nxt:
            stack.append(nxt)
        else:
            res += pairs(len(stack))
            stack = [nxt]
    else:
        res += pairs(len(stack))
    return res

assert(pairs(3) == 3)
assert(pairs(2) == 1)
assert(pairs(0) == 0)
assert(solution([5, 3, 1, 5, 5, 3]) == 4)
assert(solution([5, 5]) == 1)
assert(solution([5]) == 0)
assert(solution([]) == 0)