P/S: Before you mark or vote this question as duplicate, I wanted to post my answer here (How can I fix apt error “W: Target Packages … is configured multiple times”?), but the question has been protected, which made me to resort to asking a new question and answering it.
I just installed a new copy of Linux OS (Ubuntu 18.04) on my local machine, and I am currently installing new applications.
I have successfully installed some applications, however after installing Opera Browser, I seem to run into an error.
Each time I run the command
sudo apt update
in my terminal, I get the error
W: Target Packages (non-free/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:51 and /etc/apt/sources.list.d/opera-stable.list:4
displayed multiple times.
I have tried to make some research on how to solve this, but there seems to be no solution yet. I need some assistance.