Bind two keys which are already bound to some other action to the custom action [User10]. Here I will use 'K' (default binding [Compass]) and 'L' (default binding [Headlights]).
Run the attached mission, each time either key is pressed and immediately released two lines will be printed to chat;
<Time> User10 key pressed...
...and flagged by InputAction.
Now unbind 'L' from [HeadLights] after which pressing it will only yield;
<Time> User10 key pressed...
...unless it is held down and begins repeating. The 'K' key which is still also bound to [Compass] behaves as before.
A test for a single keystroke should not return different results just because the key doesn't happen to also be bound to some other action.