code: public void addProperty(String key, Object value) throws ProcessBagException {
          if (properties.containsKey(key)) {
              throw new ProcessBagException(key + " already used!");
          } else {
              properties.put(key, value);
          }
      }
docs: Adds a new property only if it does not exist already
  
  @throws ProcessBagException if the key is already present