what is behind the exisiting tiles , you need to think of weight , plaster takes about 16kg per m2 gyproc about 24 per m2 . double tiling will possibly take you over that ,
dont overboard existing tiles then tile , unless you can mechanically fix hardie backer to wall .
if you do tile over existing tiles , mapie do a primer called ecopirime grip. use that then a powder adhesive like weber set plus or the the likes
usually best just to strip out and board again , mostly leaves a better job. hope this helps