I messed up my AS literature and had to completely retake that subject. I went through all the classes, as well as retaking the exam. I'm also planning on retaking 2 psychology exams next year.
The highest grade that you achieve will be the one that others see. For example, I got a D in Literature the first time round, and got a B this time. The D is now irrelevant (and unspoken of, thank god).
If you feel comfortable in retaking the exams, and know that you can make time to revise and learn, then go for it. It will be stressful though, if you're anything like me, because I was constantly worried about getting a worse or the same grade, despite all the extra work.