I would doubt it major time to spend my time and effort tiling on top of old tiles. Yes, it's less work keeping the existing tiles on, but it's a downgrade if you look at the details. Think of how you would fix the tap if there's another tile layer is added. Most of the time it won't fit under the spacer, so it will look ugly. How will it look where the tiles meet the door and windows frames? Tiling over tiles is the easy way, not the professional way.