that is up to you to decide 其中be up to sb 已经是由.决定的意思了 为什么还要加

这个不奇怪,因为还可能会是to choose,to make the decision等等.另外,在需要表达决定什么时.就非要这样表达不行.例如：That is up to you to dicide whether to sue the company or not.