The problem, apparently, is with the bar at the end of the url.
Here's how the final file is:
# Original APT package sources gathered by Ansible
deb http://archive.ubuntu.com/ubuntu/ xenial main universe
deb http://archive.ubuntu.com/ubuntu/ xenial-updates main universe
deb http://archive.ubuntu.com/ubuntu/ xenial-backports main universe
# Official Ubuntu repositories
deb http://archive.ubuntu.com/ubuntu xenial main universe
deb http://archive.ubuntu.com/ubuntu xenial-updates main universe
deb http://archive.ubuntu.com/ubuntu xenial-backports main universe
# Ubuntu Security repository
deb http://security.ubuntu.com/ubuntu xenial-security main universe
Yes, the role currently compares the URLs directly, without stripping a slash at the end. This could be implemented and probably will be fixed in the DebOps monorepo at some point.
The problem, apparently, is with the bar at the end of the url.
Here's how the final file is: