#include <vector>
#include <set>
#include <algorithm>
#include <iostream>
#include <iterator>
 
using namespace std;
 
vector<int> findDublicates(vector<int> const& v)
{
    typedef std::set<int> DupSet;
    DupSet ds;
    DupSet ds2;
    vector<int> dublicates;
    for_each(v.begin(), v.end(), [&](int n) 
            {if (ds.find(n) != ds.end()) ds2.insert(n); ds.insert(n); });
    copy(ds2.begin(), ds2.end(), std::back_inserter(dublicates));
    return dublicates;
}
 
int main()
{
    vector<int> test = { 1, 2, 3, 4, 5, 1, 2, 3, 7 };
    vector<int> out = findDublicates(test);
    copy(out.begin(), out.end(), ostream_iterator<int>(cout, " "));
}

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