[Mashiro]

Are you running Debian 8 with Debian 9 repository? This is not recommended as it can bring great conflict between package version and applications. Moreover it can even break the OS by installing more up to date packages but not compatible packages. Be careful with this!

Replace:

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deb http://deb.debian.org/debian stretch main
deb-src http://deb.debian.org/debian stretch main

deb http://deb.debian.org/debian stretch-updates main
deb-src http://deb.debian.org/debian stretch-updates main

deb http://security.debian.org/debian-security/ stretch/updates main
deb-src http://security.debian.org/debian-security/ stretch/updates main

deb http://deb.debian.org/debian stretch main contrib non-free
deb-src http://deb.debian.org/debian stretch main contrib non-free

deb http://deb.debian.org/debian stretch-updates main contrib non-free
deb-src http://deb.debian.org/debian stretch-updates main contrib non-free

deb http://security.debian.org/debian-security/ stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security/ stretch/updates main contrib non-free

with this:

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deb http://deb.debian.org/debian stretch main contrib non-free
deb-src http://deb.debian.org/debian stretch main contrib non-free

deb http://deb.debian.org/debian stretch-updates main contrib non-free
deb-src http://deb.debian.org/debian stretch-updates main contrib non-free

deb http://security.debian.org/debian-security/ stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security/ stretch/updates main contrib non-free

deb https://download.webmin.com/download/repository sarge contrib

This will solve the first problem you had that reported conflicts due to duplications. Run "apt-get update" after replacing the sources to verify everything is working.


Regarding your second error. You have to install the authentication key of the custom repository you added so that your Debian server can trust the sources from which the packages are being downloaded.

Do the following to fix the issue.

Go to /tmp folder:

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cd /tmp

Download the webmin repository authentication key:

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wget http://www.webmin.com/jcameron-key.asc

Install the authentication key:

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apt-key add jcameron-key.asc

After successfull installation of the key you can delete the downloaded file:

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rm -f jcameron-key.asc

Install the HTTPS module for apt package manager:

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apt-get install apt-transport-https

Refresh package sources:

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apt-get update

That should be it .