[2020] Map broken because of duplicate blocks

I had a map ready and decided to tweak it a little more. But after replacing about 10 blocks I suddenly realized the block counter went up from 9k to 18k. When I deleted the blocks I created and nothing more, the counter, for some reason, showed 20k blocks. As a result of this, the maps is very laggy, and I've found that when I try to delete one block (support beam block), there are duplicates of it and I have to click many, many times.

Let me know if I can do anything to help. I have not found any way to replicate this.