that is normally caused by a malformed sources file.
check /etc/apt/sources.list and make sure that it matches the repos listed here:
Code:
http://docs.kali.org/general-use/kali-linux-sources-list-repositories
Failing that, run an apt-get clean && apt-get update && apt-get upgrade
or, alternatively rm -rf /var/cache/apt/archives
and then apt-get update && apt-get upgrade
If none of that works, you may have a dud mirror (bad rsync or similar) and will need to wait until they sort their stuff out. Otherwise, you can define a custom kali mirror. the procedure is here
http://docs.kali.org/kali-support/kali-linux-mirrors