If Biden had a double the double would have made an appearance and said he was dropping out.
Good point. The thing is that Biden has a chin like Dudley Do Right in some pictures and not in others. There is/was an obvious double. I have also seen pictures of him with and without ear lobes.
The CIA has been using masks since the 1980s. There is a video on YouTube of a woman who was in charge of disguises sitting in the Oval Office with GHW Bush. She had such a good mask on that Bush did not recognize her. Imagine what they have come up with in the last forty years.
I think the reason Biden did not call a press conference is that he really is incapacitated. They called a lid for the rest of the week as of today. He may have had another stroke. Which means he could be dead by the end of the week. IF he is not already.
The double could have made a video and they could use AI to generate a speech to announce he was resigning.