Um, apt-file in zesty is actually too old to trigger this, so I manually added the config file for testing it, and verified it that way: root@d5c73dbb41a2:/# apt update Hit:1 http://security.ubuntu.com/ubuntu zesty-security InRelease Hit:2 http://archive.ubuntu.com/ubuntu zesty InRelease Hit:3 http://archive.ubuntu.com/ubuntu zesty-updates InRelease Hit:4 http://archive.ubuntu.com/ubuntu zesty-backports InRelease Hit:5 http://archive.ubuntu.com/ubuntu zesty-proposed InRelease Reading package lists... Done Building dependency tree Reading state information... Done 7 packages can be upgraded. Run 'apt list --upgradable' to see them. [...] W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:46 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:46 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:48 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:48 root@d5c73dbb41a2:/# apt install libapt-pkg5.0 Reading package lists... Done Building dependency tree Reading state information... Done The following packages will be upgraded: libapt-pkg5.0 1 upgraded, 0 newly installed, 0 to remove and 6 not upgraded. Need to get 784 kB of archives. After this operation, 1024 B of additional disk space will be used. Get:1 http://archive.ubuntu.com/ubuntu zesty-proposed/main amd64 libapt-pkg5.0 amd64 1.4.8 [784 kB] Fetched 784 kB in 0s (921 kB/s) debconf: delaying package configuration, since apt-utils is not installed (Reading database ... 6656 files and directories currently installed.) Preparing to unpack .../libapt-pkg5.0_1.4.8_amd64.deb ... Unpacking libapt-pkg5.0:amd64 (1.4.8) over (1.4.6~17.04.1) ... Setting up libapt-pkg5.0:amd64 (1.4.8) ... Processing triggers for libc-bin (2.24-9ubuntu2.2) ... W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:14 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:14 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:24 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:24 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:16 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:16 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:26 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:26 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:46 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:46 W: Target Contents-deb-legacy (Contents-amd64) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:48 W: Target Contents-deb-legacy (Contents-all) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:48 root@d5c73dbb41a2:/# apt update Hit:1 http://security.ubuntu.com/ubuntu zesty-security InRelease Hit:2 http://archive.ubuntu.com/ubuntu zesty InRelease Hit:3 http://archive.ubuntu.com/ubuntu zesty-updates InRelease Hit:4 http://archive.ubuntu.com/ubuntu zesty-backports InRelease Hit:5 http://archive.ubuntu.com/ubuntu zesty-proposed InRelease Reading package lists... Done Building dependency tree Reading state information... Done 6 packages can be upgraded. Run 'apt list --upgradable' to see them.