The dual-retrieval （DR） operation sequencing problem in the flow-rack automated storage and retrieval system （AS/RS） is modeled as an assignment problem since it is equivalent to pairing outgoing unit-loads for each DR operation. A recursion symmetry Hungarian method （RSHM）, modified from the Hungarian method, is proposed for generating a DR operation sequence with minimal total travel time, in which symmetry marking is introduced to ensure a feasible solution and recursion is adopted to break the endless loop caused by the symmetry marking. Simulation experiments are conducted to evaluate the cost effectiveness and the performance of the proposed method. Experimental results illustrate that compared to the single-shuttle machine, the dual-shuttle machine can reduce more than 40% of the total travel time of retrieval operations, and the RSHM saves about 5% to 10% of the total travel time of retrieval operations compared to the greedy-based heuristic.
