One thing that Da Vinci did was to sketch his idea out, re-sketch them and continue this until he found what he was looking for. I think this is what people are seeing. He did the same thing with the chapel cieling. He sketched and sketched and sketched. You would, most likely, see the same types of things.
Maybe canvas was at a premium and he used a sheet more than once. The other thing that Da Vinci hoaxers overlook is that Da Vinci wasn’t at the Last Supper. All those people in the painting are Italian models.
OK then, how do you explain what appears to be the remarkable likeness of Mr. Bill that is burned into the toast at Jesus’ right hand?