title: How can I configure std::priority_queue to ignore duplicates?
body: <p>How can I configure <code>std::priority_queue</code> to ignore duplicates?</p>
  
  <p>When I add a key that is already contained then this new one should be ignored. (In my case, the priority for the old and the new one will always be exactly the same.)</p>
  
  <p>Complexity-wise it should not make a difference: It will try to insert at the appropriate location, find the existing one there and do nothing. The question is just if <code>std::priority_queue</code> is configurable in that way.</p>