Must old thinset be removed when replacing ceramic tile?
Just removed grout and old chipped ceramic tile from hallway. It appears tile was originally installed in thinset over concrete floor. Must all of the thinset be removed or can I set replacement tile in place over top of old hardened thinset using something like Gorilla glue before grouting?
Thinset is called an adhesive. It does more than just that, but to answer your question about Gorilla Glue, you should not use that as an adhesive. Thinset is the proper adhesive.
The second thing thinset does is provide a flat, hard base that conforms to the surface you're applying to. Theoretically, a new layer of thinset can conform to the irregularities in the current thinset. However in practice, using a notched trowel, and depending on how bumpy the surface it, it's going to be extremely difficult or impossible for you to get a consistent level of thinset down to set your tile in. The result will be air pockets (possibly resulting in cracked tile), pressure point (possibly resulting in cracked tile), and lippage (unlevel tile).
And that doesn't even take into account that the thinset that's there might not be adhered that well to the subfloor, especially after you've taken up the old tile.
Strongly recommended to fully remove the old thinset. Harbor Freight has a rather inexpensive demo hammer that comes in handy for that. But if you're lucky, it won't be particularly strong or particulary well adhered, and it will come up easily with a hammer and cold chisel without killing you. Or better yet a sturdy shovel.