Prevent individual devices to be added multiple times in the Energy configuration

[mblennegard]

When adding individual devices, for tracking energy consumption, in the Energy configuration screen, devices can now be added multiple times.

When having many devices that tracks energy usage it is not clear which devices have already been added. It would be much better if already added devices were hidden from the drop-down list with devices, alternatively to have a checkbox that you could manually add to show already added devices in case that is used by someone.