Comment 16 for bug 1743995

@ZdravkoG, my answer was for the original poster of this bug report. In you case it is unexpected to have entries for both xenial and zesty in the same sources.list
Could you please file a separate bug report. You can do it from the command line with the command:

ubuntu-bug ubuntu-release-upgrader

It's easier to separate issues and it can still be marked as duplicate if after investigation it appears to be the same.