Hello! Both sentences are not the same for the first one 'you can't undo what is done' is in the future. It is saying that whatever happens you cannot change it. The second sentence 'you can't undo what has been done' is saying that what has happened in the past as far as actions or events are unchangeable. The difference between the two is the tenses of what 'is' to come, and what 'has been' done. Hope this is helpful!