Can I install laminate (Pergo) flooring over previous underlayment?
I am sorry I don't have time to research this right now but I will give you a quick answer and maybe edit later.  First - call Pergo and ask.  

Second, I have installed 4 different types of Pergo+underlayment and all of them say to take out old underlayment.  I think there is first an issue with the floor becoming too spongy and second if there is spongy/vertical movement it could could splintering or disconnecting of the tiles.

If your old underlayment is "padding" I would remove.  If it is just a thin vapor barrier then keep it.