post PAR simulation with Xilinx Project Navigator: how?

Hi,
I'm using Xilinx ISE 6.2, and I've a small VHDL project that went
through PAR. I want to simulate the post PAR model, but I'm not sure how
to. From the Project Navigator, I did generate the post PAR simulation
model, which apparently is a vhd file containing the top level entity I
designed. How do I apply the original testbench to this entity? If I try
to add the new vhd file to the project I get an error, because an entity
with the same name already exists. Must be simple, but I don't
understand what the procedure is....


thanks,
ra

ra