Tiling on existing tiles

I've been asked to finish tiling a friends bathroom, where the new ceramic tiles have just been stuck on top of the old (ceramic) tiles. I've not done this before, nor would I out of choice, but he's been left in the lurch so I've said I'll help out. I'm putting in a new wc and cistern, so will tile before I fit those.
Anything in particular to watch out for when tiling over existing tiles? Do I need a special tile cement (it's around/behind the wc in his bathroom).
Thanks for any advice.
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DavidM wrote:

They do need to be spotlessly clean so the adhesive can stick properly. Best to stagger the joins so they dont follow the existing joins. Tiling over tiles leaves a 'thick' edge which needs something to finish the edge.
--
Dave - The Medway Handyman
www.medwayhandyman.co.uk
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