Can you install hardwood flooring over existing hardwood flooring?

It shouldn't be done. Part of the instalation process is testing for humidity and you won't be able to get an accurate reading. It should be places right on top of the concrete floor under the laminate flooring with a layer of padding and vapor barrier plastic in between. I prefer to get them as one piece, less work. Taking out the laminate is also important to identify any uneven spots in the floor and then to fix them. Taking up the laminate is a quick process with the right tool.