press Alt+F2 and type gnome-terminal if you’re using GUI
sudo nano /etc/apt/sources.list
(To understand better: in the directory
/etc you can find (nearly) all configuration files of your computer and in the
apt directory you find the
souces.list file that contains all the repositories that APT uses to update or install and upgrade software)
Edit the file removing or commenting with
# at the start of the line any duplicated rows.
Press Ctrl+X to exit and press y to save the file (or n to quit without saving).
Then update to see if you’ve fixed it: