Villagers who have already taken a job/job block will become unemployed due to another/a new villager taking up the same profession by using the same block.  When interacted with, the previously employed villager's trade GUI will instantly disappear when opened. There will then be two villagers with the same profession, however, only with the newly employed villager can the player successfully open the GUI.

In Short: Villagers will steal another villager's profession, despite there only being one profession block available. Previously employed villager outwardly retains the profession, but cannot be traded with.