villigers claming already taken job blacks

Put the summary of the bug you're having here

What I expected to happen was...:

for only 1 villager to claim 1 job block

What actually happened was...:
2 or more villagers clamed a block

1. Steps to Reproduce:
2.  put a villager and a job bock down
3. place any amount of the villagers 
4. eventually one of them will claim the already taken block