http://www.nvcc.edu/catalog/cat2013/policies/repeating.html
Here is the policy on repeating. If you take a class a second time, the grade will retroactively replace the first grade and will recalculate the GPA. If you need to take a class a third time, you can certainly see if the student service center will let you sign up, but if not, you will need to see the dean of the division for the class you are trying to get into at the campus it is held. The employees at NOVA do want to answer, but remember between mid July and late August at NVCC, it's like retail's Black Friday just about every day, but without additional employees. Please be patient with them and go to the campus to work with them and/or email your advisor or counselor. Usually they'll answer within 2 days. But for future reference, the earlier in the year, the better.