Debian/Ubuntu system update on a Plesk server shows a warning: Duplicate sources.list entry

Symptoms

• Debian/Ubuntu system update shows a warning:

  CONFIG_TEXT: Fetched 17.5 kB in 2s (6082 B/s)
  Reading package lists... Done
  N: Ignoring file 'plesk.list.ai_back' in directory
  '/etc/apt/sources.list.d/' as it has an invalid filename extension
  W: Duplicate sources.list entry
  http://autoinstall.plesk.com/debian/PMM_0.1.10/jessie/all amd64
  Packages
  (/var/lib/apt/lists/autoinstall.plesk.com_debian_PMM%5f0.1.10_dists_jessie_all_binary-amd64_Packages)
  W: You may want to run apt-get update to correct these problems

• The following warning appears during microupdate installation or running apt-get update:

  CONFIG_TEXT:W: Target Packages (all/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list.d/50sw_autoinstaller.list:4 and /etc/apt/sources.list.d/plesk-migrator.list:1

Cause

This is a Plesk bug with ID PPPM-6049 that is planned to be fixed in future updates.

Resolution

Until the issue is permanently fixed, the warning can be safely ignored.

Nevertheless, the following workaround can be performed to get rid of the warning: