A laptop with eDP-1 (which is its default screen) and a mirror monitor attached on HDMI-1 (which is only connected in certain working locations) work fine. The configuration can even be seved. However, the configuration is not automatically remembered or applied. Perhaps this has something to do with the moment the monitor is being connected, such as before boot, before unlocking the screen, during suspend or while normal operation.
LXRanR needs to keep an eye out for if a monitor gets connected or seems to be connected compared to the previous situation it assumed was applicable. The workaround is each time to click LXDE's start button / choose Preferences / choose Monitor Settings / check HDMI-1 / click apply / click OK / close the LXRandR application. these are seven steps which could be automated and provide the end-user with a more efficient user experience.