Can you install a new hardwood floor over existing hardwood floor?

Nope, you need to take out the old one, i believe.

Edit - No, I disagree with this. This depends on the situation. If you have a hardwood floor installed on top of plywood, it is generally better to remove the hardwood and replace it. But, if there is no subfloor (and this is sometimes the case in the older homes in our area), then sometimes it is actually fine to install on top of the hardwood. The hardwood (or often pine if it's older) then becomes the subfloor and then you can nail on top of that. It is important, though, that the hardwood is installed perpendicular (or diagonal to that) for better installation.

Oh, and it's important to look at heights and make sure there is enough clearance for the wood and that doors can be opened (or shaved).