Attempting to determine if a vector contains a duplicate. This applies to the absolute value of the elements in the vector. I have tested my implementation with a few cases and have gotten inconsistent results.

bool has_duplicate(vector<int> v) {
  vector<int>::iterator it;

  for (it = v.begin(); it != v.end(); ++it) {
    if (v[*it] < 0)  
      v[*it *= -1;
    if (count(v.begin(), v.end(), v[*it]) > 1)
      return true;
  }
  return false;
}

vector<int> v1 {1, -1}       // true
vector<int> v3 {3, 4, 5, -3} // true
vector<int> v2 {2, -2}       // false
vector<int> v4 {3, 4, -3}    // false
vector<int> v5 {-1, 1}       // false

Any insight on the erroneous implementation is appreciated