This actually happens a lot and I’m completely clueless as to why/how.
I will be editing my block. I’ll hide, unhide things. Next thing I know I have a duplicate of the same block instance inside of my block. What I thought worked was to click on another block inside of the editor tree… and then back into the parent block. No, that is not working. I can never tell what to keep or what is really there or not. Most of the time (I think) I end up copying and deleting the strange duplicate block and closing. Once I close, the block completely disappears and UNDO will not rescue it. For the most part, I am normally lucky because I usually end up copying my block to edit it (mostly because this happens so much OR because I need to orient to whatever I need) — So when it does make this weird duplicate and I close… and it disappears – i have my other block still surviving… WITH the updated changes… go figure. Not sure what this is all about.