Very good, you have found the MUSIC and MOVIES folders.
Back to the original question: " A lot of the music and videos in my iTunes are also in my “Movies” and “Music” folders. Must they be both places, they take up much space, I would like to have them only in iTunes"
When you say, "A lot of the music and videos in my iTunes.", exactly where do you mean. Is that when you open the iTunes application? If that is the case, then that indeed is normal. Your MUSIC folder is the source for the iTunes application.
If not, can you identify where the duplication may exist.
Ciao.