9. One or more non-transitory computer-readable media storing instructions, which, when executed by one or more processors, cause one or more computing devices to perform: identifying a subset of content items of a search index based at least upon content items returned in a result set generated by a search engine for a particular query that was previously submitted to the search engine; for each content item in the subset, determining a similarity value with respect to all other content items in the subset; upon determining that a similarity value of a first content item and a second content item of the subset of content items satisfy a particular threshold value, determining which of the first content item and second content item are less relevant to the particular query; and marking, in the search index, the less relevant to the particular query of the first content item and second content item as a duplicate content item for the particular query; in response to determining that the more relevant of the first content item and the second content item to the particular query is no longer available: determining which of the content items are marked as a duplicate and satisfy the particular threshold value is the most relevant content item to the particular query; and updating the search index by removing the mark as duplicate for the most relevant content item to the particular query.
16. The one or more non-transitory computer-readable media of claim 9 , wherein the method is performed offline prior to receiving a query from a user.