Can you install a ceramic tile floor over a ceramic tile floor?

You can do this if your current tile floor is stable. Go around to each tile and knock on it, if it sounds solid, it's stable, if it sounds hollow, it's not.

Even if everything is stable, I still wouldn't recommend installing over old ceramic floors. The main reason is that it's going to make that area of flooring really high. This will be especially noticeable if it butts up against other flooring types such as carpeting. Also, your doubling the chance of the flooring failing and needing repairs. If your OLD mortar under the first set of tile starts to fail, then it's going to affect your new floor too.

It's always best to do jobs the right way, rip out the old flooring and then install your tile.