unless you just want one. If the tiles on the concrete are holding good and tight to the concrete, you can go over them with out any problem. However, if any tile are missing or chips gone, then you can either place some tile in the areas where missing and then float out the areas where chips are to make it smooth. Use a Floor Levelor compound and just float or trowel it over that area to make it all level. No big deal.
Then lay your tile over this. I would lay the tile down so that the new tile is not directly and evenly over the old square. Set them off center so that the new tile is riding on four corners of the old tile.