Measuring duplication in search results is described. In one example, duplication between a pair of results provided by an information retrieval system in response to a query is measured. History data for the information retrieval system is accessed and query data retrieved, which describes the number of times that users have previously selected either or both of the pair of results, and a relative presentation sequence of the pair of results when displayed at each selection. From the query data, a fraction of user selections is determined in which a predefined combination of one or both of the pair of results were selected for a predefined presentation sequence. From the fraction, a measure of duplication between the pair of results is found. In further examples, the information retrieval system uses the measure of duplication to determine an overall redundancy value for a result set, and controls the result display accordingly.