Hi List, I'd like to check if any duplicate exists in my collection as below: rule "check Duplicate" when ref1: Reference ($type1: referenceType, $value1: referenceValue) ref2: Reference (this != ref1, referenceType == $referenceType, referenceValue == $value1) then errorHandler.record("Duplicate Reference is not allowed."); end Both my referenceType and referenceValue are String. I didn't get any error but it didn't work. Any idea? I am using 4.01 GA Best Regards, Tim Nguyen