Very coarse grit paper (glass paper) over everything. Take as much of the original glazed surface off the existing tiles first, as much as you can so you create a rough surface to take the new adhesive. Try to remove the old grout as much as you can to create troughs for the new adhesive to grip into.
You aren't going to see the original tiles under the new ones, so be as ruthless as you want with them, but try not to damage them so much that you loose the integrity of the wall (don't try to bash big holes through them).
Have you tried removing any of the existing tiles? What kind of wall are they on (plasterboard stud, plaster on brickwork)? Do the existing tiles look flat and smooth so you can follow their line to get a good finish on the new tiles?
A good weatherproof / waterproof / made for shower room adhesive to stick the new tiles on is OK. Please don't go for an adhesive / grout combination type stuff for a job like this. They have more grit in them than proper adhesive, and don't stick as well to a shiny surface as proper clay/silicone adhesive does.
(I prefer powder grout that you can mix yourself in small batches and so don't get things stuck in it or loose the colour, which spoils the end finish. But that's just me.)
So, to answer you question. No, there should be no problem with tiling over tiles, as long as you prepare everything properly in advance.
Have fun.