If there are no cracks in the grout or cracked tile, then you could entertain the idea of going over the old with new. The additional height relative to doorways, baseboard trim and heating units on the floor have to be taken into account. There are transitions that will slope down to another flooring type if you have this situation. The bigger issue could be the added weight if this is over wood framing, a cement slab is not an issue. You would need the services of a structural engineer to determine the dead load capacity of your wood subflooring and if the added weight of the new tile would push it past its limits.

FYI, mastic is glue in a bucket and never to be used on a floor under porcelain. It may never dry. Use a modified thinset, comes in a bag and is mixed with water.

Thanks for the advice. This is an exterior entry way on a concrete slad so the weight shouldn't be a problem. Is there anything I should do to prep the existing ceramic tile so the thinset adheres better?

Hit it with a rough grade of sandpaper on a belt sander or a palm sander to give a little better adhesion. Mop the tile with clean water to remove all the dust. Use a modified thinset to set the tile.