This happens when either you (or some software that you installed) has altered the source's of the repository.
From a clean install of backtrack 5:
Code:
root@bt:~# wc -l /etc/apt/sources.list
4 /etc/apt/sources.list
root@bt:~# cat /etc/apt/sources.list
deb http://all.repository.backtrack-linux.org revolution main microverse non-free testing
deb http://32.repository.backtrack-linux.org revolution main microverse non-free testing
#deb http://source.repository.backtrack-linux.org revolution main microverse non-free testing
deb http://updates.repository.backtrack-linux.org revolution main microverse non-free testing
root@bt:~# cat /etc/apt/sources.list.bt
deb http://all.repository.backtrack-linux.org revolution main microverse non-free testing
deb http://32.repository.backtrack-linux.org revolution main microverse non-free testing
deb http://source.repository.backtrack-linux.org revolution main microverse non-free testing
root@bt:~#
root@bt:~# ls -l /etc/apt/sources.list.d/
total 0
root@bt:~#
Using other sources other than the official repository, isn't recommended.
http://www.backtrack-linux.org/wiki/...ck_install_.3F