J Park Tiling

In theory yes it is fine to tile over tiles. there are a couple of things to consider first though, as the new tiles will be fixed to the existing tiles the quality of the job will depend entirely on how well the original tiles were bonded to the wall. ideally the grout lines should be off set from the original tiles.
secondly is the amount of weight being hung onto the wall, depending on what the original tiles were stuck to will depend on how much weight it can carry. if the original tiles were tiles onto plaster then that would give you approx 20kg per m2 of weight carrying. an average m2 of metro tiles is around 12 kg plus around 1.5-2kg of adhesive which would give approx 14kg per m2 so i would definitely consider the weight otherwise they will literally pull plaster off the walls and potentially damage something or worse case injure somebody. hope this is of help, feel free to ask if any more advice is needed.