Answer:
You sure can get highlights over highlights. The more highlights you get though, the drier and courser your hair will get. I have long brown hair I got golden highlights in June 2009 but I didn't think they were deep colored enough so I got them again in November 2009. She got them a little brighter in November which I like more but then I had gold and blonde and of course brown. Just this July 2010 I got touch up highlights and they were a mix of gold and blonde and I love them. They will keep me satisfied for a while so that way I can get my hair back to a soft silky feeling!