It explicitly says that there’s a duplicate, keep only one of the two files (/etc/apt/sources.list.d/signal-xenial.list or /etc/apt/sources.list.d/signal-desktop.list).
Run only one of these:
sudo rm -f /etc/apt/sources.list.d/signal-xenial.list
sudo rm -f /etc/apt/sources.list.d/signal-desktop.list