echo "[some repository]" | sudo tee -a /etc/apt/sources.list
The tee command is called as the superuser via sudo and the -a argument tells tee to append to the file instead of overwriting it.
Your original command failed, as the IO redirection with >> will be done as the regular user, only your echo was executed with sudo.
Calling a sudo subshell like
sudo sh -c 'echo "[some repository]" >> /etc/apt/sources.list'
works, too as pointed out by others.