prevent double filter

SpigotCore_Main/src/de/steamwar/entity/REntityServer.java

@@ -76,7 +76,10 @@ public class REntityServer implements Listener {
     }
 
     public void setCallback(EntityActionListener callback) {
+        boolean uninitialized = this.callback == null;
         this.callback = callback;
+
+        if(uninitialized)
             TinyProtocol.instance.addFilter(useEntity, filter);
     }