While trying to disable the terminal bell in Gnome on Ubuntu 18.04 I accidentally triggered a bug causing Gnome to play two alert sounds when triggered. The bug has been known (Ubuntu bug #1753070) for quite a while now, but unfortunately has not been fixed yet. In the bug report Ame Nomade describes a solution that I will repeat here so I can find if I happen to have this problem again. I a terminal type:
dconf reset /org/gnome/desktop/sound/theme-name
This resets the key to its default value.
PS: The terminal bell can be deactivated in the settings of the terminal itself, so messing with Gnome's settings is not necessary.