CEC, back button duplicates

Goto settings->system->input->peripherals->CEC adapter menu to the bottom

You can only manipulate these 3 settings for your purpose (which are finally are stored in a file at /home/osmc/.kodi/userdata/peripheral_data).

But it is up to you to find out what setting is the best for this remote control, see this thread as suggestion/motivation, only [SOLVED] Raspberry CEC - repeated inputs