Student Request Screen

Does anybody know of a way to limit a student from requesting the same elective twice? Weight lifting is a popular elective that students can only have 1 period. I have students that fill every elective slot with weights. Obviously, I can make the consequence if they request it more than once, they don't get it at all. But, I would like to find a way to eliminate the possibility of requesting any class twice, as that was only one example of the problem when students enter their own requests.