I have tiled over tile in a shower floor that was set on thick concrete. In that case, I had no choce due to access and budget limitations. Actually, I removed a prior layer of tile that had been added to the original tile floor! I used a chipper to remove the layer, used an angle grinder to make sure the glaze and remaining thinset were gone.
Then I retiled per usual. It came out really nice with no problems or loose tiles.