Raspbian - Upgrading MQTT from Jessie to Stretch and testing the upgraded MQTT

This post deals with how to upgrade MQTT from Raspbian Jessie to Raspbian Stretch and testing the upgraded MQTT.

When message similar to the one shown below appears after upgrading from Raspbian Jessie to Stretch it means you need to manually upgrade MQTT.

pi@raspberrypi:~ $ sudo apt-get update
Hit:1 http://archive.raspberrypi.org/debian stretch InRelease
Get:2 http://repo.mosquitto.org/debian stretch InRelease [11.0 kB]
Hit:3 http://mirrordirector.raspbian.org/raspbian stretch InRelease
Ign:2 http://repo.mosquitto.org/debian stretch InRelease
Fetched 11.0 kB in 5s (1,965 B/s)
Reading package lists... Done
W: GPG error: http://repo.mosquitto.org/debian stretch InRelease: The following signatures were invalid: EXPKEYSIG 61611AE430993623 Mosquitto Apt Repository <repo@mosquitto.org>
W: The repository 'http://repo.mosquitto.org/debian stretch InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
pi@raspberrypi:~ $

Raspbian - How to solve the problem of "xxx is is configured multiple times in /etc/apt/sources.list:xx and /etc/apt/sources.list.d/raspi.list:xx"

I ran into problem highlighted in yellow below when running "sudo apt-get update" on my RPI after upgrading from Raspbian Jessie to Raspbian Stretch.

pi@raspberrypi:~ $ sudo apt-get update
Hit:1 http://archive.raspberrypi.org/debian stretch InRelease
Get:2 http://repo.mosquitto.org/debian stretch InRelease [11.0 kB]
Hit:3 http://mirrordirector.raspbian.org/raspbian stretch InRelease
Ign:2 http://repo.mosquitto.org/debian stretch InRelease
Fetched 11.0 kB in 5s (2,032 B/s)
Reading package lists... Done
W: Target Packages (main/binary-armhf/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (main/i18n/Translation-en_GB) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (main/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (ui/binary-armhf/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (ui/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (ui/i18n/Translation-en_GB) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (ui/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: GPG error: http://repo.mosquitto.org/debian stretch InRelease: The following signatures were invalid: EXPKEYSIG 61611AE430993623 Mosquitto Apt Repository <repo@mosquitto.org>
W: The repository 'http://repo.mosquitto.org/debian stretch InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: Target Packages (main/binary-armhf/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (main/i18n/Translation-en_GB) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (main/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (ui/binary-armhf/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Packages (ui/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (ui/i18n/Translation-en_GB) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
W: Target Translations (ui/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:9 and /etc/apt/sources.list.d/raspi.list:1
pi@raspberrypi:~ $