There have been some cases when it looks like some sort of body doubles have been in use - Biden, for one - but then those probably are in use in the "normal" way, and have been, for more than a while, too, like when the person just would need to walk around, seem to be present, and shake hands, but the actual person needs a nap more in order to be able to sit through something more important later. And no conspiracy or actual replacement involved. Just somebody pretending to be the VIP for a short while so that the VIP can get some rest, or maybe sometimes join some sort of negotiations or similar while also having some sort of alibi for that time.

But deals, or deciding to move from one camp to another when a person sees that it might be more beneficial to him because the power is shifting, on their own, sometimes just plain blackmail making them do that etc... that kind of stuff has to be going on constantly even in more "normal" times, and now, when there actually seems to be some sort of larger shift going on, way more so.

Plus we don't know, and probably will never know, on what side ALL the players in this game actually are on, and whether they have been on the side where they have seemed to be all the time. There are moles on both sides, as well as those who switch sides, for one reason or another.