For me, I do not let them take quizzes over again. Like you mentioned, it would be unfair to the students that where prepared the first time. So, I don't think tests should be given again....unless.....there has been a break down of the learning process. An example would be that the highest grade was a 70. Then you would need to re-lecture using difference learn modes or teaching statigies to better communicate the topic. Then re-test them. Projects I do different. Since doing projects are where some of the real learning proceses occur, I let them re-do projects. Like all hands-on activities, most people do it better on the second try. One learns from mistakes. I want them to do it until they get good at it. But-- to be fair again to the people who do a good project on the first try, we average the grades. So that if one made a 70 on the original, and did it again and made a 100. The final grade for that project would be an 80. Students respond pretty well to this system. Those that really don't need to do it again, don't usually ask, and the students that should do it again, usually ask to. If the student makes below a 70.... I require that they do a re-do.