Keybind Option For „toggle underbarrel/ fire mode (when zoomed)“ is missing

I'm having the same symptoms. When I try to keybind underbarrel to my mouse wheel, it says that the key is a duplicate with Toggle underbarrel/ fire mode when zoomed. Toggle underbarrel/ fire mode when zoomed is missing, so I can't unbind it.

I have tried a clean install of the game, this did not fix the issue. 

when I installed the game on a different pc the key binding issue did not happened. 

Please fix this.....