Villagers duplicating and returning after death

Several weeks ago, my villagers started duplicating themselves and appearing roughly 2 chunks above my trading hall, outside of my base in the surrounding grounds and on the roof. This has created significant lag, and even after killing the duplicated villagers, they return as soon as you travel to the nether and return to the overworld. It appears to me this happens every time I travel to the nether and back, causing significant lag in the immediate area as well as a general server-wide lag. I eventually caved and killed all of my villagers. The problem resolved only temporarily. I logged in again recently and all of the villagers had returned and duplicate versions were once again populating the surrounding area. It has also started to affect the surrounding area. Player created iron golems in the base have been teleported to the roof and one disappeared entirely. Nearby horses, originally contained by fences, have either teleported or duplicated to be outside of the pen. Nearby tamed cats are also being teleported. Lag has made it so that the realm is nearly unplayable and I don't see a solution.