[sagher]

I earlier had this issue too. But you can refer to this answer for a possible solution to your problem because this helped me. Maybe it would help you too?
Additionally, here is what I did(with reference to the above-mentioned answer).

I copied old source.list to '/' directory as a backup.

Code: (Select All)

sudo cp /etc/apt/sources.list ~/

Now downloading new source.list and directly making it edited to original file.

Code: (Select All)

sudo wget "http://pastebin.com/raw.php?i=uzhrtg5M" -O /etc/apt/sources.list

Now updating with new lists.

Code: (Select All)

sudo apt-get update

Removing extras which had an impact/conflict with another file of the same, so we removed it to avoid the error.

Code: (Select All)

sudo rm /etc/apt/sources.list.d/ubuntu-extras.list

Now updating again for the last time.

Code: (Select All)

sudo apt-get update

This should work as it did for me. 
Credits are mentioned.

First is i have Debian 9. and i already done it. i update my source.list i ll paste here.

deb http://deb.debian.org/debian/ testing main contrib non-free
deb-src http://deb.debian.org/debian/ testing main contrib non-free

deb http://deb.debian.org/debian/ testing-updates main contrib non-free
deb-src http://deb.debian.org/debian/ testing-updates main contrib non-free

deb http://deb.debian.org/debian-security testing/updates main
deb-src http://deb.debian.org/debian-security testing/updates main

i got it recently from debian source.list generator.
debian 9 release is " testing/unstable" Sid

Added quotation tags - @sagher - please remember to add quotes when the content is not personally written by you.  And copied and pasted. @deanhills