Smart substation GOOSE, SV, MMS three-in-one network achieves full information sharing, the double star network reliability is high, the corresponding power device needs to support efficient and robust redundancy discarding algorithm. According to the PRP recommended double window drop algorithm is complex, disordered packet has low efficiency, timing considerations are not rigorous, restarting exception handling is not included, this paper summarizes the basic needs of discard algorithm, analyzes reliable discarded and safe receiving timing principle, proposes the sequence number state HASH algorithm and a single fixed sliding window method. A detailed analysis of the starting and ending points are or not in the sliding window, the receiving sequence number before or in or after the window situations, and gives the algorithm pseudo code for reference use. Robustness that exceeds specific interval, for the same sequence number, the content of the packet may not be the same; within specific interval, different packets may have the same FCS (frame check sequence). This method can assert and discard received duplicate packet quickly and accurately, which lays a solid basis for the power device of smart substation to realize PRP/HSR algorithm.