Think we may have been adding some entities twice.

src/main/java/com/gmail/nossr50/util/blockmeta/chunkmeta/HashChunkManager.java

@@ -619,12 +619,10 @@ public class HashChunkManager implements ChunkManager {
 
         List<Entity> tempSpawnedMobs = new ArrayList<Entity>(spawnedMobs);
         for (Entity entity : tempSpawnedMobs) {
-            if (entity.isDead())
-                mobsToRemove.add(entity);
-
-            if (!entity.isValid())
+            if (entity.isDead() || !entity.isValid()) {
                 mobsToRemove.add(entity);
             }
+        }
 
         spawnedMobs.removeAll(mobsToRemove);
         mobsToRemove.clear();