Each recording medium device includes a memory and a controller having controller information allocated thereto. Each recording medium device has a medium identifier allocated thereto. A duplication judgment device 500c comprises: an acquisition unit 521c acquiring first and second medium identifiers and first and second controller information of first and second recording medium devices; a judgment unit 522c judging whether the first medium identifier matches the second medium identifier, and whether the first controller information matches the second controller information; and an output unit 523c outputting duplication information indicating that the first medium identifier and the second medium identifier are duplicates, when the first medium identifier matches the second medium identifier and the first controller information does not match the second controller information.