For a few years now, I have had villagers double up on job blocks. For example, I had a librarian on a single lectern, who I leveled up to master. I come back a bit later, and find that a new librarian has taken over the job block. There are no other lecterns. This new villager is at novice level, with new trades. The other villager no longer visits the job block, and never restocks selling items. If the job block is broken, only the new villager loses the job, but when the job block is placed again, an unemployed villager will take it, and the old villager wanders, sometimes not even claiming a bed anymore. 
I have only ever had two villagers on one block. 

pictured: in the photo you can see 11 composters, and 12 farmer villagers. There are 14 total farmers in the village despite the constant number of composters