If you type any key in Double commander and it echoes twice in the screen most probably you use an old version of Lubuntu in combination IBUS or XIM. This can happen with Double Commander on old Xubuntu like preconfigured installations of mininet.
Unfortunately it is a known bug.
There are two solutions:
- edit /etc/default/im-config
- change IM_CONFIG_DEFAULT_MODE from auto to xim
- Go to language support
- turn the ‘Keyboard input method system to “none”