code: @Override
      public E set(int index, E element) {
      	if(element!=null && !contains(element)){
      		return super.set(index, element);
      	}
      	return get(index);
      }
docs: Replaces the element at the specified position in this list with
  the specified element.
  
  @param index index of element to replace.
  @param element element to be stored at the specified position.
  @return the element previously at the specified position.
  @throws    IndexOutOfBoundsException if index out of range
  <tt>(index &lt; 0 || index &gt;= size())</tt>.