You can have either one Music category that exists in both Books and Magazines, or two separate categories called Music that live in different places. What you have to decide is whether you want to have the same products in Books/Music and Magazines/Music. Having one category means you will see the same products no matter which way you get there. With two different categories, you can control which ones will be seen based on which parent category the user came from. If you put a product in both categories, "Music" would be listed twice in the tag list, so keep that in mind as well.