Could anyone tell me why do I have to install same packages in both chapter 5 and chapter 6, such as gcc, findutils and binutils?
How are they different? I've done every thing without problem until installing gcc-3.2 in chapter 6. but now I am haviing a trouble with installing gcc.
I get "make: *** [bootstrap] Error 2". Is this because gcc has been already installed in chap 5? or just a bug?

I don't get the idea of doing this. for instance in chap 5 installing gcc-3.2, book says:
patch -Np1 -i ../gcc-3.2.patch
patch -Np1 -i ../gcc-3.2-nofixincludes-2.patch
mkdir ../gcc-build

in chap 6, installing gcc, book says:
patch -Np1 -i ../gcc-3.2.patch
mkdir ../gcc-build

didn't we just appy patch or make dir already in chap 5?

In chapter 5, you are statically linking the installs, in chapter 6 you are not, in chapter 6 you are actually installing the software so that it can be used, because you need these tools (that you installed in chapt 5) to install the programs.
In chapter 6, once you have chrooted to /mnt/lfs, then you can no longer use the features of your original base (slackware, or whatever) because you are further down the dir tree. So in chapt 5 you are installing tools to enable you to install the programs properly. The patch, if i remember properly was to do with statically linking the install and fixing a bug with statically linking through install. This is not a staticlly linked install so does not need the patch.
Why you get the actual error, i do not know, but i am 98% sure it is not because of the original install
I am currently having other problems and unable to help any furthur as i have not reached that part of LFS yet, but i have read it.
Hope this clarifies a few things
Richard

have you deleted the original source, (the stuff you unpacked) because if you have not already deleted it, and then unpacked it, it will have the extra patch attached and will not work correctly i would not of thought. So delete the extracted files, re-unpack it, and then install

you should delete the source folders that you worked with in chapter 5 and start again with new ones in chapter 6, which is why you need to also apply the patch..... That's probably why gcc isn't compiling.