The Maritime Mobile Service Identity (MMSI), a unique AIS identifier, is crucial for maritime safety. However, MMSI duplication severely threatens navigation, emergency rescue, and regulation. This paper proposes an MMSI duplicate detection algorithm based on vessel trajectory constraints and dynamic statistical decision-making. The algorithm first conducts a single-point check by comparing a ship’s actual displacement with its theoretical maximum, calculated from its speed. It then uses a multi-point adaptive multi-threshold decision approach to confirm duplicate detection. A MATLAB simulation platform was used to validate the algorithm’s reliability and applicability. The method provides maritime authorities with an efficient and dependable automated solution for identifying duplicate MMSI.