Ok, its not a major Problem, but it would be nice when someday someone could fix it. 
The problem is i play with the Mouse in the Left Hand, and so i use for Shooters my NumPad to Walk, change Weapon and so Stuff and i allways DUCK on KP_Enter… it fits just right.
But Brink is the first Shooter i remember were i cant do this… everytime when ive been klicking on Duck and KP_Enter, it just assigned as normal Enter.
So i can screw my hole configuration, that i used since CS and UT and find another Button to Duck… whats kind of suxx, so it would be nice if there could be someday (as sooner as better :D) a fix.
Maybe there is a Workaround through the Config but im not realy aware about the ID Tech 4 commands and where i can find the config for the Keyboard.
Maybe somebody here is able to shortly fix it, would been nice.
sry for my bad englisch 