If you break a villager's bed but then place it (or another) in the same spot, the villager will not realize that it is a new bed, still thinking it is his own. However, the bed is still considered "open" to new villagers. This means that if a new villager is spawned, it will claim that bed as well. You can break and replace the bed as many times as you wish to get an infinite number of villagers on the same bed. If the bed is broken and replaced without any "bedless" villagers around, the villagers will view it as open and breed.