Take Q.V.RK advise. The adhesive or epoxy can surely hold the tile on top of other tiles securely if it is done properly. The surface of existing tile has to be thoroughly clean, otherwise it is possible that it will get de-laminated.

Removing old tile is easy with proper tool but then it is only thin layer if wall being hack off. Therefore, if new tile is not done properly also can cause the tile to fall off. However, I'll still go with removing old tile. If not the edge of tile like near the door will look thick and not nice I guess.