I keep getting an error message after updating that /etc/apt/sources are configured multiple times. I haven't found a command to clean the files.
I need a command that will repair this problem.

You forgot to post the error message so we are left to guess?

And also post the contents of your /etc/apt/sources.list file, being sure to surround any terminal output with "code" tags which become available when you click the "Go Advanced" button beneath the compose post window.

Do post the items asked for above. However, let me attempt to explain what is (probably) happening. /etc/apt/sources.list along with the files in /etc/apt/sources.d tell apt what repositories to use. Somewhere along the line one or more of the repositories is referenced more than once. So, if you look in sources.list (which may, by the way, be empty) and all the files in sources.d directory, all text files, by the way, you can probably identify which repo or repos are referenced more than once. There is no reason you cannot back up all these and then set them up manually into one sources.list file or at least remove the extra entries. The file format is pretty obvious if you look at it. If something gets screwed up you can always restore your backup. I don't know of a single command to do this, but they may know of something.

There are real bugs with the 19.3 see if you can do an update with a terminal sudo apt update if you find like I did that a file has changed then you will have to fix the file first this may be your problem.

I think 19.3 should be withdrawn until they fix the issues and put GIMP and VLC back.

If you have done a Timeshift roll back to 19.2 and see if you can fix it in 19.2

I am going to have to wait until I do another update to try and answer your questions. I think I may have to use a text editor and delete some of the duplicates.

If you try a terminal command like sudo apt install gimp and

If you get this error in your terminal commands

Type ‘>deb-src’ is not known on line 2 in source list /etc/apt/sources.list.d/tor.list

Change to root then

Edit that file and remove the ">" before "deb-src" on line 2.

I suspect you have this problem after you fix this it should work.

I remember this happened to me once before in Fedora.
I think I need a command to open /etc/apt/sources.list.d/additional-repositories.list:1/etc/apt/sources.list.d/additional-repositories/list:2 in a text editor like Gvim which I have installed and delete list 2. They are configured multiple times. I was hoping to use a command like autoclean or autoremove to do this task.

Yes Fedora is good I used to have it many years ago 1990 for many years until 2009. It always allowed you as root. I did hardly need to ask questions in Linuxquestions about it as it never let me down unfortunately Linuxquestions forgot me and I had to reapply so they have a different date for joining and would have been able to find what questions I had about it.

I changed to Linuxmint then as I wanted to dual boot. I tried recently again but the dual boot option does not show up but I know it will but I had to Google to find out how. If it had the installer that Linux Mint and Peppermint has - cannot remember it's name?- would recommend it.

Does it still start as root?

I understand that Linux Mint has start in root disabled.
I opened Gvim and found the /ect/apt/sources.list.d/
It had two identical files. I deleted one and am going to check if the error message is gone the next time I have an update to install.