Well, we could make it so you can only add mulitple instances of qtype like calculated, where the one question has multiple variants. Then, we also need to insure that each instance of the question in each attempt uses a different variant.
However, even when a question has one variant, you might want to add multiple copies. For example and essay question that asks "Write a reflective note on how you approached the questions in this section."
However, I take your point, so if we do implement this, we should find ways to build a UI that lets people do all the things they might reasonably want to do, while making it difficult for them to make accidental mistakes.