Hi, here's a very short and clear solution I came up with in a few seconds.
I guess somebody'll like the trick used in it.

#include <bits/stdc++.h>
using namespace std;
int n,i;
string s;
map<string,int>M;
int main(){
    //freopen("input.txt","r",stdin);
    cin>>n;
    for(i=1;i<=n;++i){
        cin>>s;
        ++M[s];
        if(M[s]>1)cout<<s<<endl,M[s]=-1e9;
    }
    return 0;
}

#include <bits/stdc++.h>
using namespace std;
string s;
map<string,int>M;
int main(){
        while (getline(cin, s)){
        ++M[s];
        if(M[s]>1)cout<<s<<endl,M[s]=-1e9;
    }
    return 0;
}
This is shorter .... Get Ac, but wrong . Weak tests.

Edited by author 12.09.2016 09:59