You may put multiple rigs on of the same type, but they also are effected by the stacking penalty, even more so then usual modules... they are even effected by modules you put on your ship. If you put 3 damage mods on your ship and then a damage rig on, it's nigh on useless as it acts as a 4th damage mod
At least thats what i've been told, same goes for any rig that acts like a module that also shares a stacking penalty.
Cap rigs however, just like cap modules suffer no stacking penalty. Hence people really like the cap rigs and powergrid rigs. Unfortunately for the damage/rof rigs they seem to take up lots more calibration points and suffer from the stacking penalty so i guess its cap rigs for the win.