These villagers seem to duplicate at random, and spawn at random.

I have a villager breeder. There were two brown-robed ones that had come from a glitch which is now patched. I have tried to kill them about 3 times. Every time I killed them, I played for a few hours after, then logged off. When I would log on again they would be back. This happened 3 times. But it is not limited to them. Other villagers seem to render at random - I only have one fletcher, one time when I logged on there were two. It was not a baby growing to an adult- it had the exact same unlocked trades as the original.  Entire sets of villagers seem to render at random. I believe this has also happened with some chickens in the area, since one keeps appearing randomly in a closed area but I have not kept tabs on them, so I am not sure. 

In these screenshots, i killed the brown-robed ones, re-logged, and noticed an entire set of different villagers. Notice in the back a librarian who is not longer present after the re-log, and two babies that weren't there before. 

 It should also be known that this farm is 3000 blocks out from spawn.