same elements in the vector

You could try something like this
#include <vector>
#include <iostream>

using namespace std;

void check( vector<int> & iv, int v ) {
vector<int>::iterator i;
i = find(iv.begin(), iv.end(), v );
if ( i != iv.end() ) {
++i;
i = find(i, iv.end(), v );
if ( i != iv.end() ) {
cout << "found " << v << " multiple times" << endl;
}
else {
cout << "found " << v << " once" << endl;
}
}
else
cout << v << " not found" << endl;
}

int main() {
vector<int> iv;

iv.push_back(7);
iv.push_back(8);
iv.push_back(7);

check( iv, 7);
check( iv, 8);
check( iv, 0);
}
Kurt

Yes I would like to remove all elements that are the same except one so that the vector does not contains only unique elements.

1) For general data, I think that Lindley's suggestion is fastest.

sort(v.begin(),v.end());
v.erase( unique(v.begin(),v.end()) , v.end() );

2) You might consider other containers that do not allow
duplicates (such as set).