How to keep only duplicates efficiently?

Given an STL vector, output only the duplicates in sorted order, e.g.,

INPUT : { 4, 4, 1, 2, 3, 2, 3 }
OUTPUT: { 2, 3, 4 }

The algorithm is trivial, but the goal is to make it as efficient as std::unique(). My naive implementation modifies the container in-place:

My naive implementation:

void not_unique(vector<int>* pv)
{
    if (!pv)
        return;

 // Sort (in-place) so we can find duplicates in linear time
 sort(pv->begin(), pv->end());

 vector<int>::iterator it_start = pv->begin();
 while (it_start != pv->end())
 {
  size_t nKeep = 0;

  // Find the next different element
  vector<int>::iterator it_stop = it_start + 1;
  while (it_stop != pv->end() && *it_start == *it_stop)
  {
   nKeep = 1; // This gets set redundantly
   ++it_stop;
  }

  // If the element is a duplicate, keep only the first one (nKeep=1).
  // Otherwise, the element is not duplicated so erase it (nKeep=0).
  it_start = pv->erase(it_start + nKeep, it_stop);
 }
}

If you can make this more efficient, elegant, or general, please let me know. For example, a custom sorting algorithm, or copy elements in the 2nd loop to eliminate the erase() call.