There's several reasons to answer questions in Magoosh twice, but let's start with the most important one: students who answer Magoosh questions 2 or 3 times increase their scores more than our students who answer our practice questions only once. There are biological and psychological reasons why this works: spaced repetition and conversion of important concepts from temporary to long-term memory are the key factors.
You might think that you'll remember the answers to questions you've already answered — and, sometimes, that's true! But it's not true most of the time. Most students forget what they learn and write within 3 days, and, because of that, redoing questions has immense value! When you redo a problem, you have to recall the concepts (or vocabulary or verbal strategies!) required to solve it, and you'll have to use your problem solving skills, which reinforces those concepts and strategies. So get ready to reset your stats and practice questions and dive back in to a fresh pool of practice tests and questions :)