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Hi Glenn,You will have to write an Apex Trigger for this purpose. On Master-Detail relationships you cannot set the Unique flag, and validation rules only look at the current record and not the other records in the database.You will want to create a before insert trigger, that checks if there is already a record with the same values. If so, you add an error to the duplicate fields and therefore prevent saving the record to the database.Hope this helps! If you need further assistance, please let me know. I'd be happy to help!Best regards,Martijn Schwärzer