Can you paste the output from the following command so that we can have a look at your software sources. Many thanks.
Code:
for i in /etc/apt/sources.list{,.d/*}; do echo file: $i; cat $i; echo ====; done
I would also suggest that, due to the diversity of Linux distros, you also tell us which Linux distro (and version) you are using. That will allow folk to give you better-targetted advice.