Provided is a stream data processing system and method for avoiding duplication of data process. The system including: an evaluation result storing unit for updating and storing a query condition evaluation result; a window evaluating unit for performing window evaluation; a data separating unit for separating data into new data and duplication input data; a reuse result extracting unit for receiving duplication input data from the data separating unit and extracting a query condition evaluation result; a query condition evaluating unit for receiving new data from the data separating unit, performing query condition evaluation and creating a query condition evaluation result; and a result organizing unit for receiving the query condition evaluation result, merging, outputting and transmitting the query condition evaluation result to the evaluation result storing unit.