If there are no cracks in the grout and no other failures, yes you "can" do it. Is height an issue? The new tile job will only be as good as what's underneath. I prefer to start clean, but your job...your money.

Scarify any glazed tile and clean well with TSP or Spic & Span.

You can, but you shouldn't. Putting new on top of old is never the right way to do the job. Chisel out the old and start with a clean slate.

If you have a solid base on the tile below and there is no height issue there is absolutely no reason you can't tile over it. If the old tile has cracks, or if the floor shows any flex at all you should start fresh.

Putting old on top of new is often done in many applications of home improvement.

I just finished tiling over tile on my bathroom where the first layer was firmly tiled to my cement slab. It came out wonderful. My toilet flange happened to be elevated to the point that my second layer of tile is at a better level than the initial layer for seating the toilet. Picture is of this bathroom with the tile going in.

The bathroom I did last summer had cracked tiles and appeared to flex a little, I took off all the old tile and the cement bed, added a layer of plywood to the subfloor and backerboard and retiled. Again, it just depends on what you are working with.