code: private void checkHasDistinctElements(Collection<T> coll) {
  	// keep track of the already seen elements
  	NonIterableSet<T> seenElems = new NonIterableSet<T>(coll.size());
  	for(T elem : coll) {
  	    if(!seenElems.add(elem)) {
  		throw new IllegalArgumentException("coll has duplicate element:" + elem);
  	    }
  	}
      }
docs: Debug code: check that "coll" does not have any duplicates