Only add entity to entity cache if ID doesn't exist (#428)

connector/src/main/java/org/geysermc/connector/network/session/cache/EntityCache.java

@@ -58,13 +58,19 @@ public class EntityCache {
     }
 
     public void spawnEntity(Entity entity) {
-        cacheEntity(entity);
-        entity.spawnEntity(session);
+        if (cacheEntity(entity)) {
+            entity.spawnEntity(session);
+        }
     }
 
-    public void cacheEntity(Entity entity) {
-        entityIdTranslations.put(entity.getEntityId(), entity.getGeyserId());
-        entities.put(entity.getGeyserId(), entity);
+    public boolean cacheEntity(Entity entity) {
+        // Check to see if the entity exists, otherwise we can end up with duplicated mobs
+        if (!entityIdTranslations.containsKey(entity.getEntityId())) {
+            entityIdTranslations.put(entity.getEntityId(), entity.getGeyserId());
+            entities.put(entity.getGeyserId(), entity);
+            return true;
+        }
+        return false;
     }
 
     public boolean removeEntity(Entity entity, boolean force) {