It is better to strip the old off and then lay the new tiles, and as Grumpy said you somehow need to know is the substrat behind the old tiles is good enough.
A lot of people do tile over old tiles for a couple of reasons.
Taking old tiles off that are rock solidly fixed to the walls can damage the substrat, making the need to make ready the walls once removed, and secondly if the old tiles have been layed well, generally you'll have a nice near perfectly level wall to tile straight onto.
Only things to consider tiling over old tiles are, it will close the room in a little depending on tile thickness, whether you notic it is another matter.
Plasterboard and plain plaster do have maximum load bearing weights to consider.
When tiling over exsisting tiles always try and lay the new tiles so they overlap atleast 2 exsisting old tiles, so the load weight of the new tile and adhesive is spread over more than just one old tile.
With the extra thickness of walls that you are going to get, look out for how they will look once butted upto door and window frames\ skirting boards etc.
Make sure old tiles are de-greased by washing them down with sugar soap, scoring the old tiles for extra grip is useful too.
Hope this helps
