apt.add_source keeps adding lines

[willirath]

On one of my Ubuntu deployments, I've found a /etc/apt/sources.list.d/nodesource.list containing the following:

deb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic maindeb https://deb.nodesource.com/node_10.x bionic main

If I remember rightly, we had connectivity problems when setting up the machine, and re-ran the TLJH setup multiple times. My understanding of https://github.com/jupyterhub/the-littlest-jupyterhub/blob/master/tljh/apt.py#L21_L36 is that it should prevent adding the source if it's already contained. But it does seem to fail at that.