Missing Number from array with duplicates

#include<bits/stdc++.h>
using namespace std;

int main(){

vector<int>v={1,3,5,3,4};
int n=v.size();

int expected_sum=n*(n+1)/2;
int actual_sum=0;
int unique_sum=0;
for(auto i:v){
actual_sum += i;
}
unordered_set<int>seen;

for(auto i:v){
seen.insert(i);
}
for(auto i:seen){
unique_sum += i;
}

cout<<"duplicate number : "<<actual_sum-unique_sum<<endl;
cout<<"missing number : "<<expected_sum-unique_sum<<endl;

return 0;
}