xmodmap add Control = Control_R adds two controls instead of just one I created

[sanjeev]

I tried to remap the control key to "apostrophe" like this

xmodmap -e "remove Control = Control_R"
xmodmap -e "keycode 48 = Control_R"
xmodmap -e 'add Control = Control_R'

xmodmap -e "keycode 109 = apostrophe quotedbl"

xmodmap -p shows

control Control_L (0x25), Control_L (0x42), Control_R (0x30), apostrophe (0x6d)
I thought I just removed Control_R. apostrophe entry is fine, why did I get
Control_R(0x30) again ? any ideas ?