code: public void addHeaderOutImpl(String key, String value)
    {
      if (headerOutSpecial(key, value)) {
        return;
      }
      
      ArrayList<String> keys = _headerKeysOut;
      ArrayList<String> values = _headerValuesOut;
      
      int size = keys.size();
      
      // webapp/1k32
      for (int i = 0; i < size; i++) {
        if (keys.get(i).equals(key) && values.get(i).equals(value)) {
          return;
        }
      }
  
      keys.add(key);
      values.add(value);
    }
docs: Adds a new header.  If an old header with that name exists,
  both headers are output.
  
  @param key the header key.
  @param value the header value.