event OrderKey{...}
event NewOrder {
   OrderKey key; //You can use anything for key as long as it is unique
   ...
}
event ValidNewOrder {
   NewOrder order;
}

monitor OrderFilter {
   dictionary < OrderKey, NewOrder > orders;
   action onload() {
      on all NewOrder() as order validateOrder(order);
   }
   action validateOrder(NewOrder order){
      if orders.hasKey(order.key) {
         print "Duplicate order!";
         print "Original: " + orders[order.key].toString();
         print "Incoming: " + order.toString();
      }
      else {
         orders.add(order.key,order);
         route ValidNewOrder(order);
      }
   }
}
 
monitor OrderProcessor {
...
   action onload() {
      on all ValidNewOrder() as valid spawn processOrder(valid.order);
   }
   action processOrder( NewOrder order ) {
   ...
   }
}