Is it a good idea to tile over tiles in a bathroom?

Yes its cheap and fast,

Not really when I see jobs like that being done I hear Clint east wood music in my ears dont be lazy pull the old one out and have the wall level.

Essentially no, it is not always recommended, and it is a practice i never do.
Over time, the tiles could lift away from the wall they are fixed too, causing a gap between them and the wall/adhesive, This will cause the tiles to be unstable on the wall.Although they may look secure, you can not be sure that they are still fixed securely. A simple way to check, is to take a coin or screwdriver, and lightly tap the tiles and listen to the sound it makes, normally if it gives off a dull thud, they would appear ok, If they give of a higher pitch thud or a hollow sound, it would indicate movement, either with the tiles, or the background itself (plaster/render/plasteboard).
Tiling on top of tile is not always recommended, but if you have no other choice, make sure the surface is thoroughly cleaned with a suitable degreasing agent to remove any residual fats and soap deposits. Then use a dedicated Primer - not pva! - and coat the tiles accordingly, you might want to add a small amount of adhesive to the primer mix, to give a textured gritty finish to help with the adhesion the new tiles.
Then when you begin to tile the walls, make sure that wherever you sart, the new grout lines will not run directly on top of the existing tiles.!!!
But as i said at the start, i do not recommend this way of fixing wall or floor tiles because you can not be 100% sure that they are still fixed in position or the wall underneath is good enough to take the excess weight.
Not many people are aware that plasterboard, render, blocks and brickwork have a maximum bearing load when it comes to fixing wall tiles!!
So my advice is, chop off ALL the tiles, repair the walls if needed and start with a blank canvass!!

can be done, but would not do unless good reason

If existing tiles remain in good condition and are firmly fixed to the substrate, it is possible to tile over them. A lot will depend on type of old tile and style of new tile. You also need to bear in mind how far out the new tile line will be after fixing to existing tiles with correct adhesive, especially by door trims and skirting boards. A good tip to test if old tiles are soundly fixed to the original substrate, is to tap each tile with a pound coin. If fixed soundly, the tile should have a solid sound. If a hollow sound is heard, the tile is not fixed properly and should not be tiled over! If in doubt, get an experienced tiler to check it out for you.

It is possible to tile over tiles in a bathroom depending on how solid the tiles are that are being laid over. If the tiles are of a sound nature then I would use neat/slurry primer then use a flexible powdered adhesive to fix the top layer of tile. Finished by a flexible grout

Would be best to strip the wall back taking off old tiles ,bored and re-plaster the wall for a better surface to work onto with your new tiles for a straight and professional looking.

It is not a good idea to tile over tiles, especially in a bathroom. I would recommend you to remove the old tiles and start again.

No,that's a bad idea! You need to take out the old tiles,clean the walls and after that retailing.

Not really, you better off removing the existing tiles, make good the walls and floors and start again.
Thanks

It is possible. Care would need to be taken to ensure the extra layer of tiles will fit in cosmetically, that is for instance they would not stick out more than a door frame. Also, the old tiles would need to be comprehensively cleaned. I would not recommend it though as the old adhesive under the old tiles may not take the weight of both tile layers and there is simply no way of checking that the old adhesive will take the weight.

depends on how the old tile is fixed to the wall if its still a solid fix its fine ive done it many times

Absolutely fine. Prime old tiles with acrylic primer. Then fix new tiles.

Really, No

Possible, absolutely yes.
Advisable, no. However, as a tiler myself and a professional, it is a cheaper way to upgrade a room. But any good tiler would never recommended doing this. Because then the job becomes a 'make over', rather than a professional job done.