Keyboard modifier Ctrl+Alt = Super

I am using Debian Squeeze with a USB keyboard.

Although i set up some shortcuts modifier to (Alt+Ctrl), but it also can 
be activated using Windows (Super) key. For example if I set:
key "Alt+Ctrl+t"  xterm
key "Super+Ctrl+f"  firefox

=> Super+t will open xterm
=> Super+f will open firefox (no need of Ctrl)
It is same problem for predefined shortcuts (Super+[number] -> change 
workspace)

System shortcuts still work fine: Alt+Ctrl+F1 switch to first virtual 
terminal but Super+F1 can not.

Does anybody have same this issue?

PS.: In DWM, modifier Super+Ctrl work as expected

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